HLL Performance improvement: remove useless call to Math.floor

In RegisterSet the bucket was computed as follows:

   (int) Math.floor(position / LOG2_BITS_PER_WORD)

Since position is a positive int, the int -> double -> int convertion and the call to
Math.floor were superfluous and costly. I observed a 100% performance boost on merge
operations, and 33% on offerHashed(int).
Add RegisterSet.updateIfGreater to factorize code and improve performance

A common operation for HLL and HLL+ is to conditionaly update a position if the
new value is greater than the current one. Apply the DRY principle and get a small
performance boost in return (up to 10% for merge & offer).
Add RegisterSet.merge to allow efficient merging.

A dedicated merge function with an easily predictable memory access pattern
allows a huge speedup for merge operation. It is 10 times faster than merging
using updateIfGreater.
Faster RegisterSet.merge

Up to 45% faster on sandy bridge & 30% on core2.
Merge pull request #35 from cykl/faster_hll

Faster hll