lvmetad: rework command connection setup and checking

lvmetad: rework command connection setup and checking

The lvmetad connection is created within the
init_connections() path during command startup,
rather than via the old lvmetad_active() check.

The old lvmetad_active() checks are replaced
with lvmetad_used() which is a simple check that
tests if the command is using/connected to lvmetad.

The old lvmetad_set_active(cmd, 0) calls, which
stopped the command from using lvmetad (to revert to
disk scanning), are replaced with lvmetad_make_unused(cmd).