In theory we could allow a db_write-using plugin to have other
hooks/commands by embargoing their other responses until the exclusive
period is over. That would be nice for a 'dbmirrorinfo' command, for
example.
The other option would be to *always* go exclusive on a db_write-using
plugin, so responses can never get intermingled.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>