READONLY: only send a control to schedule fast-vacuuming from child context iff we...

READONLY: only send a control to schedule fast-vacuuming from child context iff we have a connection open to the main daemon

there are some child processes where we do not create a connection to the main daemon (switch_from_server_to_client()) because it is expensive to set up and we normally might not need to talk to the daemon at all via a domainsocket.
but we might want to still call to ctdb_ltdb_store() from such chil processes.