Squashed 'src/leveldb/' changes from e991315..9094c7f

9094c7f Temporarily revert to writing .sst files instead of .ldb

git-subtree-dir: src/leveldb
git-subtree-split: 9094c7ffe45e0bedd84ea414828f768fd49bcdf7
Merge src/leveldb changes: temporarily revert to writing .sst files.
Merge pull request #3555

060fdb4 Squashed 'src/leveldb/' changes from e991315..9094c7f (Pieter Wuille)