Fix hardcode number issue in replicate.StartBackup implementation

[zhaohaidao]

Problem
Given timeout value is not used in replicate.StartBackup implementation

// From: https://github.com/siddontang/go-mysql/blob/master/replication/backup.go#L45
for {
    // The second arg should be timeout, not a hard code number
    ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
    ....
}

PS: I am curious how to test my code without travis ci environment.

[siddontang]

to make the test more stable, I think you should use a larger timeout here to ensure StartBackup timeout can work.