vacuumlazy.c: prefer the term "cleanup lock".
The term "super-exclusive lock" is an acceptable synonym of "cleanup
lock". Even still, switching from one term to the other in the same
file is confusing. Standardize on "cleanup lock" within vacuumlazy.c.
Per a complaint from Andres Freund.
Branc
Update high level vacuumlazy.c comments.
Update vacuumlazy.c file header comments (as well as comments above the
lazy_scan_heap function) that were largely written before the
introduction of the HOT optimization, when lazy_scan_heap did far less,
and didn't actually prune during its initial heap p
