Adding merge functions to RocksDBJava

Summary:
Added support for the merge operation to RocksJava.
You can specify a merge function to be used on the current database.
The merge function can either be one of the functions defined in
utilities/merge_operators.h, which can be specified through its
corresponding name, or a user-created function that needs to be
encapsulated in a JNI object in order to be used. Examples are
provided for both use cases.

Test Plan: There are unit test in MergeTest.java

Reviewers: ankgup87

Subscribers: vladb38

Differential Revision: https://reviews.facebook.net/D24525
Integrated feedback from ankgup87

Test Plan: tested using unit tests

Reviewers: ankgup87

Differential Revision: https://reviews.facebook.net/D24573