autovacuum: Improve wording in a couple places

autovacuum: Improve wording in a couple places

A few strings (one WARNING and some memory context names) in the
autovacuum code were written in a world where "worker" had no other
possible meaning than "autovacuum worker", but that's long time gone.
Be more specific about it.

Also, change the WARNING from elog() to ereport(), to add translability.

Author: Bharath Rupireddy <[email protected]>
Reviewed-by: Nathan Bossart <[email protected]>
Reviewed-by: Justin Pryzby <[email protected]>
Reviewed-by: Kyotaro Horiguchi <[email protected]>
Reviewed-by: Dilip Kumar <[email protected]>
Reviewed-by: Masahiko Sawada <[email protected]>
Discussion: https://postgr.es/m/CALj2ACX2UHp76dqdoZq92a7v4APFuV5wJQ+AUrb+2HURrKN=NQ@mail.gmail.com