opal_thread integration with libevent

[npe9]

This is an initial stab at changing the Open MPI libevent integration to use opal threads instead of pthreads for alternative threading libraries.

Currently, although opal threads are now replaceable via an mca, opal_event_use_threads() in event.h is just a macro passthrough to libevent's pthread event locking api. This was causing event list corruption with alternative thread libraries.

This commit factors out libevent threading and integrates it with opal thread api. The changes are currently untested. The initial changes are primarily to solicit feedback on my approach before I begin more extensive testing and debugging.