[rs] rework `wrpc-transport` crate

[rvolosatovs]

wrpc-transport crate is a bit of a mess with leaky abstractions - it served well to reach an MVP with NATS transport, but it's time to rework it to better suit this projects needs.
Largely this should be based on wrpc Go package design
The biggest requirements here are:

• handshake and accept logic should be moved into the individual transport implementations, wrpc-transport should just operate on plain AsyncRead and AsyncWrite handles with indexing
• AsyncValue and AsyncTransmission should be removed (#1)
• #2

[rvolosatovs]

This actually done now in #81