Fix gateway query sending in transport lib

[kalabukdima]

https://github.com/subsquid/sqd-network/blob/main/crates/transport/src/actors/gateway.rs#L257

Queries are put to the queue and then moved to another internal queue. If this internal queue is full, the query is just dropped, and there is no way to know about it on the caller side.

1. Introduce some kind of backpressure and ensure that "Outbound message queue full" errors don't occur anymore.
2. Sanitize interface for send_query — return channel and hide the details about id matching.