Optimize multiple set (multi-set or mset bulk update) with setq

The Java client, and others, implement a more efficient bulk loading feature when multiple updates are sent to the cluster at one time. They use setq to send multiple requests without needing to round-trip the result after each one.

The .NET client does not implement this; it only implements a multi-get operation. The multi-set is important for efficiently loading data.

This probaby doesn't have to be Couchbase-specific, but would apply to vanilla memcached servers, too.