light: always use designated primary as sync source
marsadm: remove superfluous connect links
marsadm: prevent mutual overwrite of delete symlinks

Add an (unused) host context. Previously, concurrent
log-delete operations triggered by cron could lead to
unnecessary loss of deletetion symlinks.
light: prevent races between deletions and re-erected objects
marsadm: wait for deletions after leave-resource
marsadm: reject invalidate earlier when no primary is designated
marsadm: do resource checks only once per command
marsadm: early abort is-device-on when not designated primary