[#148] Modifications to add, poll and peek in Merger

[vaibhav-yb]

This diff adds the following modifications to the above mentioned functions:

1. add() now updates the tablet safetime on adding every message
2. peek() now returns a message if the commit time of the front message is less than or equal to the stream safetime.
3. Asserts to detect any state which may lead to inconsistency, such asserts would throw an AssertionError and will cause the connector to stop now.
4. Unit tests for the added functions.

[vrajat]

Will there be a new PR for asserts?

[vaibhav-yb]

Will there be a new PR for asserts?

Yes @vrajat There will be a separate PR for asserts and dumps as it requires me to merge all three of the existing changes first since asserts cover Merger and Messages both.

It is now added as a part of the same PR.

[vrajat]

For testing asserts, it should be sufficient to add two messages out of order in the same tablet? That should fire the assert (that you may add) in setTabletSafeTime.

Two messages that have the same commitTime but out of order like COMMIT followed by BEGIN should trigger the other assert in add.

[vrajat]

Couple of minor comments. Ack on taking up the config parameter in a different PR