remove dependency on libhashkit by simply embedding the two hashing types we require

Currently, libcouchbase relies on libhashkit, though it doesn't really need the entire thing. Since libhashkit is only part of libmemcached, it'd be good to decouple this, and simply have the crc32 and ketama hashing available to us in libcouchbase.

I think the crc32 dependency is actually from libvbucket, but in any event, this issue is to track removing that dependency.