Rename logical decoding's pg_llog directory to pg_logical.

The old name wasn't very descriptive as of actual contents of the
directory, which are historical snapshots in the snapshots/
subdirectory and mappingdata for rewritten tuples in
mappings/. There's been a fair amount of discussion what would be a
good name. I'm settling for pg_logical because it's likely that
further data around logical decoding and replication will need saving
in the future.

Also add the missing entry for the directory into storage.sgml's list
of PGDATA contents.

Bumps catversion as the data directories won't be compatible.