walletdb: refactor Read, Write, Erase, and Exists into non-template func

Summary:
In order to override these later, the specific details of how the Read,
Write, Erase, and Exists functions interact with the actual database
file need to go into functions that are not templated.

This is a backport of core#19292

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9618