Allow callback to change size of existing value. Change return type of the callback function to an enum status to handle 3 cases.

Summary:
This diff fixes 2 hacks:
* The callback function can modify the existing value inplace, if the merged value fits within the existing buffer size. But currently the existing buffer size is not being modified. Now the callback recieves a int* allowing the size to be modified. Since size is encoded as a varint in the internal key for memtable. It might happen that the entire value might have be copied to the new location if the new size varint is smaller than the existing size varint.
* The callback function has 3 functionalities
    1. Modify existing buffer inplace, and update size correspondingly. Now to indicate that, Returns 1.
    2. Generate a new buffer indicating merged value. Returns 2.
    3. Fails to do either of above, based on whatever application logic. Returns 0.

Test Plan: Just make all for now. I'm adding another unit test to test each scenario.

Reviewers: dhruba, haobo

Reviewed By: haobo

CC: leveldb, sdong, kailiu, xinyaohu, sumeet, danguo

Differential Revision: https://reviews.facebook.net/D15195