Add DynamicMemoryUsage() to LevelDB

Summary:
Backport of Bitcoin Core PR12604 and PR12784
https://github.com/bitcoin/bitcoin/pull/12604
https://github.com/bitcoin/bitcoin/pull/12784

Test Plan:

make check

Run bitcoind with -debug=leveldb and check the log file for the LEVELDB category and the following pattern:
WriteBatch memory usage: db=..., before=...MiB, after=...MiB
Verify that printed sizes (before and after) make sense.

Reviewers: Fabien, O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3948