host/logmux: Ensure the DB is open when persisting sinks
host/logmux: Ensure the DB is open when persisting sinks

This was causing flynn-host to panic on shutdown when it had closed the
DB but then tried to persist the logaggregator sink once it was stopped.

Signed-off-by: Lewis Marshall <748e1641a368164906d4a0c0e3965345453dcc93@lmars.net>

philiplb/flynn,philiplb/flynn,lmars/flynn,lmars/flynn,lmars/flynn,flynn/flynn,lmars/flynn,philiplb/flynn,flynn/flynn,philiplb/flynn,flynn/flynn,lmars/flynn,philiplb/flynn,flynn/flynn,flynn/flynn