Batch apply dml events

[shlomi-noach]

This PR applies DML events (events read from the binary logs) in batches, i.e. it aggregates events from the binary log and writes multiple DMLs (insert, delete, update) in a single transaction.

This has the potential to increase gh-ost's write capacity. This would potentially help #348, though we have not yet diagnosed #348 enough.

The PR introduces a --dml-batch-size flag, default 10.

Noteworthy about the implementation: gh-ost does not wait for events to arrive in order to batch. The logic is as follows:

• If there's nothing in the event queue (coming from the binary log), copy some rows
• If there's an event to handle in the events queue, read it. Further determine how many more available events are in the queue. Of these, read at most dml-batch-size. Of these, only handle the prefix that is true DML events (distinguished from gh-ost's internal events).

So the logic doesn't block on waiting for "more events to arrive", but is greedy, and takes as many available events as there are, up to some limit, and only in suh case that there are events to apply.

[shlomi-noach]

This was subjected to prod testing for the past days and behaves as expected; merging.