Add totempg_threaded_mode_enable() api

This API allows totem to operate as a multithreaded library. Performance is
better without threads but some library users may only have multithreaded
systems. In the corosync case where we have removed threads, this reduces
cpu utilization by ~10% by removing about 50% of the mutex lock and unlock calls
that occur during typical operation. Since the latest corosync is nearly
thread free, there is no need for mutex operations.

Signed-off-by: Steven Dake <sdake@redhat.com>
Reviewed-by: Angus Salkeld <asalkeld@redhat.com>