omcache: new features and major refactoring to get closer to first release

New functionality to get closer to being able to actually use this:

* Responses are always passed to a response callback (if defined)
* Add omcache_get operation for easily reading data

Removed some unnecessary (for v0.1) features complicating the code base and
cleaned up last year's code drop a lot:

* Distribution algorithm is always Ketama with Jenkins' One-at-a-time hash
* Merged the relevant bits of oconst.c into omcache.c
* Removed replication support for now, it needs a real spec and use case
  before it can be reintroduced
* Split memcache commands to commands.c, omcache.c only has the core now
* Added memcached_protocol_binary.h with all the required definitions to
  remove an external dependency to a header file with multiple versions