Extend RocksDB to also support asynchronous read operations

[tillrohrmann]

Right now, our RocksDB abstraction supports asynchronous writes to the underlying RocksDB database. What it currently lacks is support for reading data (synchronously as well as asynchronously). Instead when reading data we are directly accessing the underlying db object:

restate/crates/rocksdb/src/lib.rs

Line 118 in 41e606e

/// todo: remove this once all access is migrated to this abstraction

. It would be great to extend RocksDB to also support asynchronous read operations. This is a prerequisite for letting multiple PartitionProcessors run on the same Tokio runtime. Otherwise, blocking RocksDB read operations might block shared Tokio threads.

[AhmedSoliman]

This also needs to include iterators (including log-servers)