Having pg_shadow cache use pg_hba.conf cache code

I want to change the code that maintains the pg_shadow password cache
into the format used by pg_hba.conf so they can all use the same code.
This will allow me to cleanly use the pg_shadow code for pg_group cache
too because the pg_shadow cache has binary searching for lookups. I
will merge the binary search stuff into the new code. New format would
be:

"user" "password" "valid until"

Objections?

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