Upgrade to Lucene 4.8.1

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