Refactoring postmaster's code to cleanup after child exit

Reading through postmaster code, I spotted some refactoring
opportunities to make it slightly more readable.

Currently, when a child process exits, the postmaster first scans
through BackgroundWorkerList to see if it was a bgworker process. If not
found, it scans through the BackendList to see if it was a backend
process (which it really should be then). That feels a bit silly,
because every running background worker process also has an entry in
BackendList. There's a lot of duplication between
CleanupBackgroundWorker and CleanupBackend.

Before commit 8a02b3d732, we used to created Backend entries only for
background worker processes that connected to a database, not for other
background worker processes. I think that's why we have the code
structure we have. But now that we have a Backend entry for all bgworker
processes, it's more natural to have single function to deal with both
regular backends and bgworkers.

So I came up with the attached patches. This doesn't make any meaningful
user-visible changes, except for some incidental changes in log messages
(see commit message for details).

--
Heikki Linnakangas
Neon (https://neon.tech)