msg/async: rename ms_event_ -> ms_async_, drop unused config options

Signed-off-by: Sage Weil <sage@redhat.com>
msg/Makefile: keep headers

Signed-off-by: Sage Weil <sage@redhat.com>
msg/Messenger: allow ms_type 'random'

Randomly select simple or async, for testing purposes.

Signed-off-by: Sage Weil <sage@redhat.com>