Make Zebra send notfound, and use received notfound to finish requests

[teor2345]

Motivation

zcashd and Zebra don't actually need notfound messages:
https://github.com/zcash/zcash/blob/e7b425298f6d9a54810cb7183f00be547e4d9415/src/main.cpp#L5632
https://github.com/ZcashFoundation/zebra/blob/main/zebra-network/src/peer/connection.rs#L115

But Zebra should send notfound as much as possible, so that inv routing works efficiently.
This also helps with testing #2156.

We might also want to use them for SPV clients - does the Zcash network have any of those?

Suggested Solution

• send notfound so Zebra peers know that we've replied with all the transactions/blocks we have
• make sure Zebra checks for notfound when it's waiting for more blocks or transactions (we might already do this)

Rejected Alternatives

zcashd doesn't do this, and it isn't really supported by the protocol.

• send notfound to tell peers when our mempool is empty
  • make Zebra check for notfound when waiting for a MempoolTransactionIds reply, rather than timing out because there is no reply

[teor2345]

This is required for #2156 - Zebra should send notfound as much as possible, so that inv routing works efficiently.

[mpguerra]

@teor2345 can you please add your estimate to this issue when you get a chance, probably no need to do a planning poker session with the whole team since you're already working on it