Re: Some other things about contrib/bloom and generic_xlog.c

> So how do we look at a generic log record, say "ok, the upstream that wrote
> this says it needs to invoke registered generic xlog hook 42, which is
> <func> from <extension> at <ptr>" ?

Record enough information in WAL that the rmgrs can have names instead
of ID numbers.  Stick the list of extension rmgrs in use into each
checkpoint record and call it good.