Squashed 'src/leveldb/' changes from 13da2d6758..53a1914e00

53a1914e00 Merge bitcoin-core/leveldb-subtree#65: refactor: use inline constexpr over static const in headers
811ba7c883 refactor: use inline constexpr over static const in headers

git-subtree-dir: src/leveldb
git-subtree-split: 53a1914e0040541e5ab6efc19748fc20d5c821ce