Minor cleanup around checking datachecksum worker result

Minor cleanup around checking datachecksum worker result

Rename the 'success' field in DataChecksumState to 'worker_result'.
That's more appropriate when it's not a simple boolean.

Don't access the field after releasing the lock in ProcessDatabase().
No other process should be modifying it, but if we bother to do any
locking in the first place, let's do it right.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/b283fbb9-298e-4953-9120-eefaf24fae20@iki.fi