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
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