RCBC-46 Adding Couchbase::ConnectionPool

Primarily to facilitate an option for ActiveSupport::Cache::CouchbaseStore
to use a connection pool in place of monitor locks for thread safety.
Thread safe locks are still the default and have been moved into their
own module.

Change-Id: I0e844b39f9b26bb5845b82a9050381d54f030dbb
Reviewed-on: http://review.couchbase.org/26028
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>