transformer: add acculumator transformer

This adds a transformer accumulating counters until a threshold, and then
flushing them out.

This implements a solution to blueprint swift-batched-requests

Change-Id: Ic1f36138d8ee1e5705f2285987763fbff9de0184
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "transformer: add acculumator transformer"