This commit upgrades to the latest Lucene 4.8.1 release including the
following bugfixes:
* An IndexThrottle now kicks in when merges start falling behind
limiting index threads to 1 until merges caught up. Closes#6066
* RateLimiter now kicks in at the configured rate where previously
the limiter was limiting at ~8MB/sec almost all the time. Closes#6018