The primary motivation is to deconflate the leading `F`s in `Failure` and
`Forward`. In particular, when using a generics, we used `F` for forward, which
could easily be confused for `F` for `Failure`. This resolves the conflation.
This is the async analog of 'rocket_sync_db_pools', rewritten to be
cleaner, leaner, easier to maintain and extend, and better documented.
Resolves#1117.
Resolves#1187.