walletdb: moveonly: Move BerkeleyBatch Cursor and Txn funcs to cpp

Summary:

Put the implementation in the cpp, not the h file.

Partial backport (4/5) of core PR19290:
https://github.com/bitcoin/bitcoin/pull/19290/commits/8f033642a8c6874184e297b97b951b9bd12ffd75

Depends on D8613.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8614