go livesql/binlog: set Localhost in binlogSyncerConfig

This sets the localhost field of the BinlogSyncerConfig. If unset,
as it was, it will use the os.Hostname() value as the hostname instead.

In rare situations, the OS's hostname could be too long for the slave
host's "host" field, and we got an error like:

  panic: ERROR 1105 (HY000): Failed to register slave: too long 'report-host'

which would crash the server. So we strip the hostname to be 60
characters, which is the max allowed length.

It doesn't seem like the hostname of the slave hosts matters that much;
Verified everything still worked as expected. We could hardcode
"localhost" here as the host name but are keeping the OS's hostname in
case it's useful for debugging on remote hosts.

To see the hostname of slave hosts:
mysql> show slave hosts;
+------------+--------------------------+------+-----------+------------+
| Server_id  | Host                     | Port | Master_id | Slave_UUID |
+------------+--------------------------+------+-----------+------------+
| 2926146535 | Arthur-Huang-MacBook-Pro | 3308 |         1 |            |
| 4101280766 | Arthur-Huang-MacBook-Pro | 3308 |         1 |            |
|  738891311 | Arthur-Huang-MacBook-Pro | 3308 |         1 |            |
| 3490062834 | Arthur-Huang-MacBook-Pro | 3308 |         1 |            |
| 3176772553 | Arthur-Huang-MacBook-Pro | 3308 |         1 |            |
|  985189460 | Arthur-Huang-MacBook-Pro | 3308 |         1 |            |
| 4240725287 | Arthur-Huang-MacBook-Pro | 3308 |         1 |            |
| 2752132824 | Arthur-Huang-MacBook-Pro | 3308 |         1 |            |
| 3221796892 | Arthur-Huang-MacBook-Pro | 3308 |         1 |            |
| 3038601891 | Arthur-Huang-MacBook-Pro | 3308 |         1 |            |
+------------+--------------------------+------+-----------+------------+

Also see go-mysql-org/go-mysql#388

livesql/binlog.go

@@ -7,6 +7,7 @@ import (
 	"encoding/binary"
 	"errors"
 	"fmt"
+	"os"
 	"reflect"
 	"strings"
 	"sync"
@@ -100,9 +101,21 @@ func NewBinlogWithSource(ldb *LiveDB, sourceDB *sql.DB, host string, port uint16
 		return nil, err
 	}
 
+	// Slave hosts have a max hostname length of 60 characters.
+	const maxHostNameLength = 60
+	localHostName, err := os.Hostname()
+	if err != nil {
+		localHostName = "localhost"
+	}
+	if len(localHostName) > maxHostNameLength {
+		runes := []rune(localHostName)
+		localHostName = string(runes[0:maxHostNameLength])
+	}
+
 	syncer := replication.NewBinlogSyncer(&replication.BinlogSyncerConfig{
 		ServerID: binary.LittleEndian.Uint32(slaveId),
 		Host:     host,
+		Localhost: localHostName,
 		Port:     port,
 		User:     username,
 		Password: password,