acl-plugin: time out the sessions created by main thread too (VPP-948)

In multithread setup the main thread may send packets,
which may pass through the node with permit+reflect action.
This creates the connection in lists for thread0,
however in multithread there are no interupt handlers there.

Ensure we are not spending too much time spinning in a
tight cycle by suspending the main cleaner thread
until the current iteration of interrupts is processed.

Change-Id: Idb7346737757ee9a67b5d3e549bc9ad9aab22e89
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
(cherry picked from commit c1ff53f25d04ec1cc31844abd38014e91e398b5f)