[Java] Temporary set the number of BG threads based on the number of BG compactions.

Summary:
Before the Java binding for Env is ready, Java developers have no way to
control the number of background threads.  This diff provides a temporary
solution where RocksDB.setMaxBackgroundCompactions() will affect the
number of background threads.

Note that once Env is ready.  Changes made in this diff should be reverted.

Test Plan:
make rocksdbjava
make jtest
make jdb_bench
java/jdb_bench.sh

Reviewers: haobo, sdong

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18681