Summary:
As discussed in #2742 , this pull-requests brings the iterator's [SeekForPrev()](https://github.com/facebook/rocksdb/wiki/SeekForPrev) functionality to the java-api. It affects all locations in the code where previously only Seek() was supported.
All code changes are essentially a copy & paste of the already existing implementations for Seek().
**Please Note**: the changes to the C++ code were applied without fully understanding its effect, so please take a closer look. However, since Seek() and SeekForPrev() provide exactly the same signature, I do not expect any mistake here.
The java-tests are extended by new tests for the additional functionality.
Compilation (`make rocksdbjavastatic`) and test (`java/make test`) run without errors.
Closes https://github.com/facebook/rocksdb/pull/2747
Differential Revision: D5721011
Pulled By: sagar0
fbshipit-source-id: c1f951cddc321592c70dd2d32bc04892f3f119f8
044a71e27e