Refactor mapFileUseCount increment and decrement to separate functions
AddRef and RemoveRef
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/19334 | core#19334]] [2/4]
https://github.com/bitcoin/bitcoin/pull/19334/commits/71d28e7cdca1c8553531bb3a4725d7916363ec5c
Depends on D10022