Cancel in-flight RPC requests

[realbigsean]

Description

Deneb introduces delayed-lookup logic, where we listen on gossip for a set period of time before triggering an RPC request. We're observing quite frequently that blocks/blobs arrive on gossip eventually and our RPC requests only serve to waste download. I think it'd be very beneficial to cancel in-flight RPC requests.

from @AgeManning

It would be pretty easy to cancel a request because we open a stream for each request. We can just drop the stream and it will cancel.

I can make a public function you can use to cancel requests if its useful

[realbigsean]

No longer necessary due to #4992