reimplement-leveldb-in-rust

reimplement leveldb in rust, with some new features.

Features

• Search tired levels(L0 currently) in parellel;
• Gather writes from multiple threads at the same time into a single wal;
• Metrics and prometheus exporter;
• MVCC enabled.

Plans

• Support the async feature: provide the async versions of each interface;
• Support the Skiplist memtable;
• Support the dump and scan interfaces, and support manual compactions;
• More detailed metrics;
• Support the compare-and-set primitives;
• Support other features that are interesting in rocksdb.