RPC server: investigate whether some RPC calls could benefit as `blocking` a.k.a `tokio::spawn_blocking`

[niklasad1]

jsonrpsee supports three different types on calls sync, async and blocking

The methods has just been ported how they were defined via https://github.com/paritytech/jsonrpc so just sync or async but some RPC calls that are sync I/O such as reading DB might facilitate from using tokio::spawn_blocking instead of tokio::spawn which is used for the calls defined as async RPC calls.

See paritytech/jsonrpsee#486 for further information

[niklasad1]

So indeed, it looks like almost all state_calls should be blocking as those hasn't any awaits points and would mess with the scheduler, some of those calls are really slow and the public RPC nodes.

/cc @lexnv would be good to know if you could share your findings after investigating state_getKeys.