use two different lock files

hi

i think it would be better to use two different lock files
one for the snapshot state
and a second one for the transfer-to-target state

reason:
if you add a new target and you are running hourly btrbk it can happen
that the transfer is still ongoing and no new hourly snapshots are created
during this timewindow.

with two lock files the second instance of btrbk would only do the snapshoting
and after all old transfers are done the new snapshots would be moved
automatically later

thanks!