walletdb: Add MakeBatch function to BerkeleyDatabase and use it

Summary:
Instead of having WalletBatch construct the BerkeleyBatch, have
BerkeleyDatabase do it and return a std::unique_ptr<BerkeleyBatch>

This is a backport of core#19325 [2/2]
https://github.com/bitcoin/bitcoin/pull/19325/commits/b82f0ca4d5465b36debb6c57f335bdccf4899c49

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10003