cma: improve serialization of destroy and event processing

cma: improve serialization of destroy and event processing

WinOF testing with slightly different scheduler and verbs
showed some issues with cleanup. Add better protection around
destroy and event processing thread.

Remove destroy flag and add refs counting to conn objects
to block destroy until all references are cleared. Add
locking aroung ref counting and passive and active
event processing.

Signed-off-by: Arlin Davis <arlin.r.davis@intel.com>