Some bugs have surfaced stemming from recent work on this initiative.
Most come down to sloppiness on my part. The worst of the bunch involves
`erc-insert-done-hook' being narrowed on date-stamp insertion, which
defies a tacit agreement to the contrary. A related bug concerns members
of the new internal date-stamp hook possibly running twice if the latter
has a buffer-local value.

I've also added a new helper for deleting inserted messages. It attempts
to respect user markers and invisibility props.