Remove vchDefaultKey and have better first run detection

Summary:
Removes vchDefaultKey which was only used for first run detection.
Improves wallet first run detection by checking to see if any keys
were read from the database.

This will now also check for a valid defaultkey for backwards
compatibility reasons and to check for any corruption.

Keys will stil be generated on the first one, but there won't be
any shown in the address book as was previously done.

Backport core's PR10952

Depends on D1049

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1051