Update to murmur3 which is significantly faster on x86_64 systems

The author of the murmur hash algorithm has improved the implementation (https://code.google.com/p/smhasher/wiki/MurmurHash3).  Bitcask's load time is bounded by the time it takes to load the keydir (read: create an in-memory khash of the keys stored in the casks by reading the hint files from disk, this uses the murmur hash function).  A faster hash function should reduce start-up time for large data sets when I/O isn't the bottleneck.