MySQL listener doesn't support proxy protocol

[r0mant]

This causes issues when Teleport is behind a load balancer with proxy protocol feature enabled, e.g. this is the case for Cloud customers.

In such cases mysql client fails to connect with an error like:

➜  mysql --defaults-group-suffix=_xxx
ERROR 2026 (HY000): SSL connection error: error:00000000:lib(0):func(0):reason(0)

And there are following logs on the proxy:

ERRO [DB:PROXY]  Failed to handle MySQL client connection. error:[
ERROR REPORT:
Original Error: *errors.fundamental invalid sequence 88 != 1
Stack Trace:
	/go/src/github.com/gravitational/teleport/lib/srv/db/mysql/proxy.go:119 github.com/gravitational/teleport/lib/srv/db/mysql.(*Proxy).performHandshake
	/go/src/github.com/gravitational/teleport/lib/srv/db/mysql/proxy.go:57 github.com/gravitational/teleport/lib/srv/db/mysql.(*Proxy).HandleConnection
	/go/src/github.com/gravitational/teleport/lib/srv/db/proxyserver.go:163 github.com/gravitational/teleport/lib/srv/db.(*ProxyServer).ServeMySQL.func1
	/opt/go/src/runtime/asm_amd64.s:1374 runtime.goexit
User Message: invalid sequence 88 != 1] db/proxyserver.go:165