LU-9480 lnet: add discovery thread

LU-9480 lnet: add discovery thread

Add the discovery thread, which will be used to handle peer
discovery. This change adds the thread and the infrastructure
that starts and stops it. The thread itself does trivial work.

Peer Discovery gets its own event queue (ln_dc_eqh), a queue
for peers that are to be discovered (ln_dc_request), a queue
for peers waiting for an event (ln_dc_working), a wait queue
head so the thread can sleep (ln_dc_waitq), and start/stop
state (ln_dc_state).

Peer discovery is started from lnet_select_pathway(), for
GET and PUT messages not sent to the LNET_RESERVED_PORTAL.
This criterion means that discovery will not be triggered by
the messages used in discovery, and neither will an LNet ping
trigger it.

Test-Parameters: trivial
Signed-off-by: Olaf Weber <olaf@sgi.com>
Signed-off-by: Amir Shehata <amir.shehata@intel.com>
Change-Id: I38a48ab7f61c8ef1b994cd17069729f243912bdf
Reviewed-on: https://review.whamcloud.com/25786
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>