Switch to FNV-1a hashing

This algorithm has better avalanche characteristics that FNV-1 while
being just as fast.
Inline FNV-1a hash algorithm

The algorithm is so simple there's no need to incur the performance
overhead over using hash/fnv package and the associated Write
interface.  This change makes hashKey 40% faster than before.  That's
a 72% improvement over what we had a few commits ago.