go livesql/binlog: set Localhost in binlogSyncerConfig

This sets the hostname of the slave hosts to be "localhost". 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.

Before this change:
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 |            |
+------------+--------------------------+------+-----------+------------+

After this change:
mysql> show slave hosts;
+------------+-----------+------+-----------+------------+
| Server_id  | Host      | Port | Master_id | Slave_UUID |
+------------+-----------+------+-----------+------------+
|  186920018 | localhost | 3308 |         1 |            |
| 2071718504 | localhost | 3308 |         1 |            |
| 3278845420 | localhost | 3308 |         1 |            |
| 2121800266 | localhost | 3308 |         1 |            |
| 3721418580 | localhost | 3308 |         1 |            |
| 3544321535 | localhost | 3308 |         1 |            |
|   50012544 | localhost | 3308 |         1 |            |
| 4141694557 | localhost | 3308 |         1 |            |
| 1374039930 | localhost | 3308 |         1 |            |
+------------+-----------+------+-----------+------------+

It doesn't seem like the hostname of the slave hosts matters that much;
Verified everything still worked as expected.

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

livesql/binlog.go

@@ -103,6 +103,7 @@ func NewBinlogWithSource(ldb *LiveDB, sourceDB *sql.DB, host string, port uint16
 	syncer := replication.NewBinlogSyncer(&replication.BinlogSyncerConfig{
 		ServerID: binary.LittleEndian.Uint32(slaveId),
 		Host:     host,
+		Localhost: "localhost",
 		Port:     port,
 		User:     username,
 		Password: password,