pipeline: manager publish multiple counters

This makes the polling agent publish all counters in a row.
This fixes bug #1126990 and bug #1130475.

This moves the publisher() method to the *manager*. No agent/middleware
interacts with only one pipeline, this one an implementation mistake.

Change-Id: I45246849830066e39491f762b457adbdfa8d0e2e
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "pipeline: manager publish multiple counters"