MB-7863 Don't link to waited process in wait_for_process.

Linking means that the process that we wait for may be killed or
receive unexpected message (in case it traps exists) if we die or if
the process is remote and the connection between two nodes gets lost.

Change-Id: Ic560c7f94311ccb4917b38ccb51aaf4b05f9d4c8
Reviewed-on: http://review.couchbase.org/26165
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>