Optimize the lock time in the cutOver phase

We found that gh-ost always requires a lock time of 1 second or even longer during the cutOver phase, regardless of whether the database is busy.

Log:
```
Done waiting for events up to lock; duration=975.482481ms
```

This indicates that the write lock duration on the change table is approximately 1 second.

The investigation revealed that the source of the long-term lock holding was time.Sleep().

Source code:

```
func (this *Migrator) executeWriteFuncs() error {
...
for {
select {
case eventStruct := <-this.applyEventsQueue:
{
if err := this.onApplyEventStruct(eventStruct); err != nil {
return err
}
}
default:
{
select {
case copyRowsFunc := <-this.copyRowsQueue:
{
...
}
default:
{
// Hmmmmm... nothing in the queue; no events, but also no row copy.
// This is possible upon load. Let's just sleep it over.
this.migrationContext.Log.Debugf("Getting nothing in the write queue. Sleeping...")
time.Sleep(time.Second)
}
}
}
}
}
}
```

The presence of `time.Sleep(time.Second)` causes the `waitForEventsUpToLock` function in the `cutOver` phase to be unable to quickly obtain the binlog processing completion notification, resulting in an excessively long locking time.

Because the binlog processing completion notification is in the `this.applyEventsQueue`, it is ultimately triggered by the execution of the `this.onApplyEventStruct()` function.

I tried removing `time.Sleep` and running `gh-ost` again to get the lock time.

```
Done waiting for events up to lock; duration=9.305327ms
```

Thanks