walletdb: Introduce AddRef and RemoveRef functions

Summary:
Refactor mapFileUseCount increment and decrement to separate functions
AddRef and RemoveRef

This is a backport of core#19334 [2/4]
https://github.com/bitcoin/bitcoin/pull/19334/commits/71d28e7cdca1c8553531bb3a4725d7916363ec5c

Depends on D10022

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10023