make relay unit more robust

[csuzhangxc]

Feature Request

Is your feature request related to a problem? Please describe:

Currently, we need the following assumptions to make relay unit workers correctly:

1. master always sends all binlog events in its binlog file
2. master sends an event until all of the previous events sent

But these assumptions may not always satisfied.

Describe the feature you'd like:

We can change the assumption to the following:

• master always sends all binlog events a slave needed

In other words, the correctness of the relay unit relies on the sending behavior of the master, not binlog file on the master.

In practice, we can use some dummy events to fill the relay log file if no corresponding events received from the master.