Fix inconsistencies for v12

Hello hackers,

Please also consider fixing the following inconsistencies found in new
v12 code:

1. AT_AddOids - remove (orphaned after 578b2297)
2. BeingModified ->TM_BeingModified (for consistency)
3. copy_relation_data -> remove (orphaned after d25f5191)
4. endblock -> endblk (an internal inconsistency)
5. ExecContextForcesOids - not changed, but may be should be removed
(orphaned after 578b2297)
6. ExecGetResultSlot - remove (not used since introduction in 1a0586de)
7. existedConstraints & partConstraint -> provenConstraint &
testConstraint (sync with implementation)
8. heap_parallelscan_initialize -> remove the sentence (changed in c2fe139c)
9. heap_rescan_set_params - remove (orphaned after c2fe139c)
10. HeapTupleSatisfiesSnapshot -> HeapTupleSatisfiesVisibility (an
internal inconsistency)
11. interpretOidsOption - remove (orphaned after 578b2297)
12. item_itemno -> iter_itemno (an internal inconsistency)
13. iterset_is_member -> intset_is_member (an internal inconsistency)
14. latestRemovedxids -> latestRemovedXids (an inconsistent case)
15. newrode -> newrnode (an internal inconsistency)
16. NextSampletuple -> NextSampleTuple (an inconsistent case)
17. oid_typioparam - remove? (orphaned after 578b2297)
18. recoveryTargetIsLatest - remove (orphaned after 2dedf4d9)
19. register_unlink -> register_unlink_segment (an internal inconsistency)
20. RelationGetOidIndex ? just to remove the paragraph (orphaned after
578b2297)
21. slot_getsomeattr -> checked in slot_getsomeattrs ? (an internal
inconsistency and questionable grammar)
22. spekToken -> specToken (an internal inconsistency)
23. SSLdone -> secure_done (sync with implementation)
24. stats_relation & keep_buf - remove (orphaned after 9a8ee1dc & 5db6df0c0)
25. SyncRequstHandler -> SyncRequestHandler (a typo)
26. table_needs_toast_table -> table_relation_needs_toast_table (an
internal inconsistency)
27. XactTopTransactionId -> XactTopFullTransactionId (an internal
inconsistency)

The separate patches for all the defects (except 5) are attached. In
case a single patch is preferable, I can produce it too.

Best regards,
Alexander