Results 4 issues of dorianzheng

Can we inline large value at the highest level for improving range scan performance at the cost of some potential write performance regression.

Since the write callback is removed by #121 , it's possible to generate SST files of valid keys and to ingest these SST files by `IngestExternalFile`

https://github.com/pingcap/grpc-rs/blob/1c64b3d16ffcbfc37efcd060cfa03a8ad69f7f46/src/server.rs#L499 If server fork more than one grpc-poll thread, server may destruct before all the grpc-poll thread exist. @overvenus

Bug

If the `map` is using the same hash function of the shard, then the hash space of the map will be less than expected because the tailing bits of a...