Batch operations pipeline

This patch allows to use a context manager to batch operations without
using the 'multi' methods. The multi methods are still more efficient,
but the pipeline allows batching operations of different types.

Change-Id: Ie614357bb7f0f8a8bd10909450b16ecdf170c162
Reviewed-on: http://review.couchbase.org/29638
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>