Partially done a big overhaul of connection management.
Redis connections are now divided into read/write categories.

This is not a finished commit.

Also fixed a bug related to the 500 error page (if there's a problem with translations, now shutdown errors are labelled as 'hard error' which doesn't do any translation).
Moved SQL connection pools into a "pools" array in "sql".
Updated all the remaining entity definition.php files.
Applied the remaining fallback connection changes to the record and link drivers.

This has not yet been thoroughly debugged yet. Just a quick test.

Yet to be implemented: the default entity cache and db engine config option.