Squashed 'src/leveldb/' changes from 9094c7f..7924331

42dcc7e Merge upstream LevelDB 1.17.
e353fbc Release LevelDB 1.17
269fc6c Release LevelDB 1.16
REVERT: 9094c7f Temporarily revert to writing .sst files instead of .ldb

git-subtree-dir: src/leveldb
git-subtree-split: 79243314e40ac31d79c68e5658a1d6a64800d50b
Merge src/leveldb changes for LevelDB 1.17.
Merge pull request #4161

938b689 Squashed 'src/leveldb/' changes from 9094c7f..7924331 (Pieter Wuille)