Use [20]bytes as map keys

I'm using the SHA-1 hash of the key most everywhere. This cuts down
significantly on GC pause times by removing the only pointers from the
biggest heap map.
Change to uint32s for offsets and sequence numbers

This makes RecordRef be 8 bytes, rather than 16, greatly reducing the
size of the refCache.
Get rid of two pointer sources in memCache

- Store timestamps as int64s
- Use a Record rather than *Record as the map value
Merge pull request #1 from cespare/gc-pause-times

Reduce GC pause time