Flow removal callbacks are executed by Sim threads

This patch establishes a channel between the FlowController and the
Simulation threads, through which the former sends flow removal
callbacks so the latter can execute them. This allows for
synchronization to be removed from several data structures.

Change-Id: Ibb46585ec1b61b1d7a041d2aff2258837cc4a16e
Signed-off-by: Duarte Nunes <duarte@midokura.com>
Merge "Flow removal callbacks are executed by Sim threads"