wallet: detecting duplicate wallet by comparing the db filename.

Summary:

Fix crash attempting to load the same wallet with different path strings
that resolve to the same absolute path.

More details here:
https://github.com/bitcoin/bitcoin/pull/14552

Backport of core PR14552:
https://github.com/bitcoin/bitcoin/pull/14552/files

Depends on D4885.

Test Plan:

ninja check
./test/functional/test_runner.py wallet_*

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4886