Improve compatibility with memcached interface

- Increment/Decrement return the new value of the counter
- Increment/Decrement with uint64 instead of int64
- Increment/Decrement drop support for floats
- Decrement stops at 0 on underflow.
- Delete now returns whether or not the key was found/deleted
- Define ErrKeyExists & ErrCacheMiss so that callers can test for those
errors in particular.