Reinstate read-only lock on hooks access in dialHook to fix data race

[LINKIWI]

Previously, in #3088, I removed the mutex guarding the implementation of dialHook in order to resolve an unbounded contention failure mode, that had the potential to backpressure commands indefinitely during periods of server downtime.

However, this introduced a data race regression, which was the original motivation of introducing the lock, in #2814.

A minimal reproduction is as follows:

package main

import (
	"context"
	"fmt"

	"github.com/redis/go-redis/v9"
)

type h struct{}

func (h *h) DialHook(next redis.DialHook) redis.DialHook {
	return next
}

func (h *h) ProcessHook(next redis.ProcessHook) redis.ProcessHook {
	return next
}

func (h *h) ProcessPipelineHook(next redis.ProcessPipelineHook) redis.ProcessPipelineHook {
	return next
}

func exec() {
	ctx := context.Background()
	opts := &redis.Options{
		MinIdleConns: 5,
	}

	client := redis.NewClient(opts)
	client.AddHook(&h{})

	fmt.Println(client.Ping(ctx))
}

func main() {
	exec()
}

package main

import (
	"testing"
)

func TestExec(t *testing.T) {
	exec()
}

$ go test -v -race -count=1 ./cmd/...
=== RUN   TestExec
==================
WARNING: DATA RACE
Write at 0x00c0000f6130 by goroutine 8:
  github.com/redis/go-redis/v9.(*hooksMixin).chain()
      /home/kiwi/sync/code/external/go-redis/redis.go:126 +0x128
  github.com/redis/go-redis/v9.(*hooksMixin).AddHook()
      /home/kiwi/sync/code/external/go-redis/redis.go:117 +0x1b1
  github.com/redis/go-redis/v9/cmd.exec()
      /home/kiwi/sync/code/external/go-redis/cmd/main.go:31 +0xaa
  github.com/redis/go-redis/v9/cmd.TestExec()
      /home/kiwi/sync/code/external/go-redis/cmd/main_test.go:8 +0x1c
  testing.tRunner()
      /usr/lib/go/src/testing/testing.go:1690 +0x226
  testing.(*T).Run.gowrap1()
      /usr/lib/go/src/testing/testing.go:1743 +0x44

Previous read at 0x00c0000f6130 by goroutine 9:
  github.com/redis/go-redis/v9.(*hooksMixin).dialHook()
      /home/kiwi/sync/code/external/go-redis/redis.go:183 +0x8c
  github.com/redis/go-redis/v9.(*hooksMixin).dialHook-fm()
      <autogenerated>:1 +0x8f
  github.com/redis/go-redis/v9.newConnPool.func1()
      /home/kiwi/sync/code/external/go-redis/options.go:516 +0x9a
  github.com/redis/go-redis/v9/internal/pool.(*ConnPool).dialConn()
      /home/kiwi/sync/code/external/go-redis/internal/pool/pool.go:213 +0x16c
  github.com/redis/go-redis/v9/internal/pool.(*ConnPool).addIdleConn()
      /home/kiwi/sync/code/external/go-redis/internal/pool/pool.go:143 +0x54
  github.com/redis/go-redis/v9/internal/pool.(*ConnPool).checkMinIdleConns.func1()
      /home/kiwi/sync/code/external/go-redis/internal/pool/pool.go:126 +0x2e

Goroutine 8 (running) created at:
  testing.(*T).Run()
      /usr/lib/go/src/testing/testing.go:1743 +0x825
  testing.runTests.func1()
      /usr/lib/go/src/testing/testing.go:2168 +0x85
  testing.tRunner()
      /usr/lib/go/src/testing/testing.go:1690 +0x226
  testing.runTests()
      /usr/lib/go/src/testing/testing.go:2166 +0x8be
  testing.(*M).Run()
      /usr/lib/go/src/testing/testing.go:2034 +0xf17
  main.main()
      _testmain.go:45 +0x164

Goroutine 9 (running) created at:
  github.com/redis/go-redis/v9/internal/pool.(*ConnPool).checkMinIdleConns()
      /home/kiwi/sync/code/external/go-redis/internal/pool/pool.go:125 +0x6d
  github.com/redis/go-redis/v9/internal/pool.NewConnPool()
      /home/kiwi/sync/code/external/go-redis/internal/pool/pool.go:109 +0x25e
  github.com/redis/go-redis/v9.newConnPool()
      /home/kiwi/sync/code/external/go-redis/options.go:514 +0x35d
  github.com/redis/go-redis/v9.NewClient()
      /home/kiwi/sync/code/external/go-redis/redis.go:665 +0x208
  github.com/redis/go-redis/v9/cmd.exec()
      /home/kiwi/sync/code/external/go-redis/cmd/main.go:30 +0xa4
  github.com/redis/go-redis/v9/cmd.TestExec()
      /home/kiwi/sync/code/external/go-redis/cmd/main_test.go:8 +0x1c
  testing.tRunner()
      /usr/lib/go/src/testing/testing.go:1690 +0x226
  testing.(*T).Run.gowrap1()
      /usr/lib/go/src/testing/testing.go:1743 +0x44
==================
ping: PONG
    testing.go:1399: race detected during execution of test
--- FAIL: TestExec (0.00s)
FAIL
FAIL	github.com/redis/go-redis/v9/cmd	0.009s
FAIL

This race is caused by concurrent access to hs.current when the connection pool executes dialHook in the background (when MinIdleConns > 0) while AddHook also mutates hs.current. However, within dialHook, only read access is required. This PR proposes fixing this by changing the mutex to a sync.RWMutex and guarding only the access to hs.current with the lock, which both solves the data race and does not regress the connection contention unit test introduced in #3088.

With this patch, the example test above passes with the race detector enabled:

$ go test -v -race -count=1 ./cmd/...
=== RUN   TestExec
ping: PONG
--- PASS: TestExec (0.00s)
PASS
ok  	github.com/redis/go-redis/v9/cmd	1.010s

[LINKIWI]

@ofekshenawa Would you be able to help with the review on this one? This is a follow up to #3088. Thanks.