Added user-defined flags to the connector

Flags are currently used by the connector to identify the type of the payload, the compression and the serialization. While memcached support at least 16 bits for the flags, only 8 bits are used by the connector. So this leaves us 8 bits to user-land to play with.

The only minor incompatible change is $cas_token in get and getMulti can now be passed by value too instead only by reference. It will only enable CAS if $cas_token is passed by reference.

The following function call won't trigger a warning anymore that parameter 3 should be passed by reference, but instead it won't activate CAS and just return the $udf_flags:

$udf_flags = null;
$memcache->get($key, null, null, $udf_flags);

Please let me know what you think of it. We're using it to enable versioning of keys and we have some other use cases as well.