NCBC-736: Add support for bulk operation Remove

Motivation
----------
Complete the API with bulk operation Remove, as we already have Upsert
and Get bulk operations.

Modifications
-------------
Added three bulk Remove operations to IBucket signature, implementation
similar to corresponding operations Get and Upsert.

Implemented all three in both CouchbaseBucket and MemcachedBucket.

Implemented tests for all three new signatures, both types of buckets.

Result
------
Remove operation can now be done in bulk, on a list of keys, with some
control over parrallelism. New tests that control this behavior pass.

Change-Id: I4c9a70af77e247c5cea4a34140e5a699fb418a23
Reviewed-on: http://review.couchbase.org/44185
Tested-by: Simon Baslé <simon@couchbase.com>
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>