Added merge operators to RocksJava

Added support for the merge operation to RocksJava as support for
specifying 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 which needs to be
encapsulated in a JNI object in order to be used. Examples are
provided for both use cases.

Signed-off-by: Vlad Balan <vlad.gm@gmail.com>