worker: New library for breaking a daemon into multiple processes.

worker: New library for breaking a daemon into multiple processes.

ovs-vswitchd is effectively a "soft real-time" process, because flows that
do not get set up quickly lead to packet loss or retransmission.  We've
done our best to keep it from blocking unnecessarily, but some operations
unavoidably block.  This new library allows a daemon to break itself up
into a main process and a worker process, connected by an RPC channel,
with the idea being that the main process will delegate any possibly
blocking operations to the worker.

This commit also modifies ovs-vswitchd to start a worker process, but it
does not actually introduce any uses for the worker process.  Upcoming
commits will add those.

Signed-off-by: Ben Pfaff <blp@nicira.com>