improved performance.

- Switch from sha1 to fnv, a cryptographic hash is bad for this
- Pre-initialize the shard to make the Concurrent Map itself lock free
- Reduce the default number of shard to 32 (even this is a lot)
- The shards are arranged in an array instead of a map for better performance
  and less memory usage
- Don't pre-size the internal shard maps, it's a waste of memory
- Removed Clear, just create a new map