async shell

[tzemanovic]

We should come up with a strategy of what needs to be changed to take advantage of async to process ABCI requests.

I think we could:

• In tower-abci split consensus requests (which need to be applied and respond in the same order) from all other requests
• Split out consensus service as a sync loop over consensus requests preserving the order (i.e. what the shell does for all the requests atm)
  • The WASM execution in consensus service can have direct sync (read-only) access to the DB
  • Block are written to the DB
• Provide async API for the DB (this could potentially use secondary instance in read-only mode https://github.com/facebook/rocksdb/wiki/Secondary-instance)
• Implement async service for all the other connections
  • dry-run tx's are sent via Info queries, they can be executed via tokio::spawn_blocking and have either direct access to a read-only secondary instance or via messages to a worker (wasmer currently doesn't support yielding from WASM (Yielding from host calls wasmerio/wasmer#1127, when it does, we could make more of this async)
  • storage read queries call the async DB API

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