Support MergeOperator in Java

[`MergeOperator`](https://github.com/facebook/rocksdb/blob/master/include/rocksdb/merge_operator.h) functionality is not currently available in the [Java API](https://github.com/facebook/rocksdb/blob/master/java/src/main/java/org/rocksdb/MergeOperator.java). [`StringAppendOperator`](https://github.com/facebook/rocksdb/blob/master/java/src/main/java/org/rocksdb/StringAppendOperator.java) is the only implementation that is exposed as part of Java API today. There has been considerable interest from the community recently to expose full functionality of MergeOperator so that they can take advantage of it in Java as well (Ex: https://github.com/facebook/rocksdb/issues/1988 , https://github.com/facebook/rocksdb/pull/2289)

cc: @adamretter