This commit fixes a bug was originally discovered during the
galera_nbo_sst_slave mtr test for 10.6 branch. However it is
relevant for all versions and can lead to intermittent SST
crashes via rsync on very fast server restarts - when a new
SST process (for example, after starting a new server instance)
overlaps the old SST process started by the previous, already
terminated server. This overlap can result in the new rsync
being killed instead of the old rsync, or the pid file from
the new rsync being killed, which then lead to problems.