Replace RocksDB by LMDB

* Removed RocksDB from Cargo.toml and database module.
* Added `heed` crate to Cargo.toml.
* Reimplemented all the database API to make use of LMDB over tokio blocking tasks.
* Fixed potential key calculation bug.
* Added documentation.
* Refactored `Database::chunk_exists` to directly load the queried chunk into cache if it exists.
* Refactored `Database::batch_insert` to avoid querying database for loading into cache chunks the function already have allocated.

Author: Asurar0