LU-12537 lnet: Sync the start of discovery and monitor threads

LU-12537 lnet: Sync the start of discovery and monitor threads

The discovery thread starts up before the monitor thread so it may
issue PUTs or GETs before the monitor thread has a chance to
initialize its data structures (namely the_lnet.ln_mt_rstq). This can
result in an OOPs when we attempt to attach response trackers to MDs.

Introduce a completion to synchronize the startup of these threads.

Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: I5d7356269090d8cbd1eab59fa29bee7ef211832f
Reviewed-on: https://review.whamcloud.com/35478
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>