Rename PGPROC->vacuumFlags to statusFlags

Rename PGPROC->vacuumFlags to statusFlags

With more flags associated to a PGPROC entry that are not related to
vacuum (currently existing or planned), the name "statusFlags" describes
its purpose better.

(The same is done to the mirroring PROC_HDR->vacuumFlags.)

No functional changes in this commit.

This was suggested first by Hari Babu Kommi in [1] and then by Michael
Paquier at [2].

[1] https://postgr.es/m/CAJrrPGcsDC-oy1AhqH0JkXYa0Z2AgbuXzHPpByLoBGMxfOZMEQ@mail.gmail.com
[2] https://postgr.es/m/20200820060929[email protected]

Author: Dmitry Dolgov <[email protected]>
Reviewed-by: Álvaro Herrera <[email protected]>
Discussion: https://postgr.es/m/20201116182446.qcg3o6szo2zookyr@localhost