Flush stale column families

Summary:
Added a new option `max_total_wal_size`. Once the total WAL size goes over that, we make an attempt to flush all column families that still have data in the earliest WAL file.

By default, I calculate `max_total_wal_size` dynamically, that should be good-enough for non-advanced customers.

Test Plan: Added a test

Reviewers: dhruba, haobo, sdong, ljin, yhchiang

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18345
Fix signed/unsigned compare
[Java] Add static method RocksDB.loadLibrary()

Summary:
Add static method RocksDB.loadLibrary() which loads necessary library
files.

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

Reviewers: haobo, dhruba, sdong, ankgup87, rsumbaly, swapnilghike, zzbennett

Reviewed By: ankgup87

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18375