Some event system fixes for the plab proxy and testing enhancements to tbsend.

* event.c / event.h

Fix assertion failure that appears to be caused by notifications coming in
on a subscription that is pending deletion (via async API call).

Also add idle polling to the ops endpoint connection to help maintain
connectivity (will do failover).

Add function to set failover flag on connection handle.

* tbsend.c

Add ability to set connection retry, failover, idle polling, and perform
looped and triggered testing.