JCBC-210: Throw CancellationException when future is cancelled.

This changeset makes sure CouchbaseClient memcached operations
behave the same way as they do in MemcachedClient. In addition,
the semantics of HttpFuture have been aligned to OperationFutures.

Change-Id: I0ce3f7fc6a40e99987c3bc32c24113d6ea4c0714
Reviewed-on: http://review.couchbase.org/24658
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>