Squashed 'thirdparty/src/leveldb/' content from commit 803d692

git-subtree-dir: thirdparty/src/leveldb
git-subtree-split: 803d69203a62faf50f1b77897310a3a1fcae712b
Merge commit '11328b5890d150b6de56e942802f1c45cc60fd6d' as 'thirdparty/src/leveldb'
build thirdparty module: leveldb.
build with leveldb.
refactor UserDbHelper::UniformBackup/UniformRestore.
memory: save cache when beginning the next query.
implement level_db.
load maintenance modules on demand.
move userdb_recovery_task to dict module.
upgrade legacy user dbs.
README: declare dependency LevelDB.
move tree_db into legacy module.