PCT-637: Query Analytics does not handle MySQL Restart

http://tools-ci.percona.com/job/percona-agent-branch/28/console
I just looked into implementation. `sync.Stop()` runs `sync.StopChan <- true` on unbuffered channel so if nothing is currently listening on `sync.StopChan` (e.g. goroutine locked) then it's going to block forever and will block also whole manager. Shouldn't be there something like this:
```go
func (sync *SyncChan) Stop() err {
    select{
    case sync.StopChan <- true:
    case <-time.After(10 * time.Second):
        return fmt.Errorf("Stop() timeouted")
    }
}
return nil
```

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