Merge leveldb subtree

Summary:
For review:

Backport of Bitcoin Core PR13925
https://github.com/bitcoin/bitcoin/pull/13925

Test Plan:

make check

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4006