[wallet] Fix leak in CDB constructor

Summary:
Now using a std::unique_ptr, the Db instance is correctly released
when CDB initialization fails.
The internal CDB state and mapFileUseCount are only mutated when
the CDB initialization succeeds.

Backport of core PR11492
Complete T220

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2231