First step in implementing #4386 (#4437)

* First step in implementing https://github.com/paritytech/polkadot/issues/4386

This PR:

- Reduces MAX_UNSHARED_UPLOAD_TIME to 150ms
- Increases timeout on collation fetching to 1200ms
- Reduces limit on needed backing votes in the runtime

This PR does not yet reduce the number of needed backing votes on the
node as this can only be meaningfully enacted once the changed limit in
the runtime is live.

* Fix tests.

* Guide updates.

* Review remarks.

* Bump minimum required backing votes to 2 in runtime.

* Make sure node side code won't make runtime vomit.

* cargo +nightly fmt

polkadot/node/network/protocol/src/request_response/mod.rs

@@ -95,7 +95,7 @@ pub const CHUNK_REQUEST_TIMEOUT: Duration = DEFAULT_REQUEST_TIMEOUT_CONNECTED;
 /// This timeout is based on what seems sensible from a time budget perspective, considering 6
 /// second block time. This is going to be tough, if we have multiple forks and large PoVs, but we
 /// only have so much time.
-const POV_REQUEST_TIMEOUT_CONNECTED: Duration = Duration::from_millis(1000);
+const POV_REQUEST_TIMEOUT_CONNECTED: Duration = Duration::from_millis(1200);
 
 /// We want timeout statement requests fast, so we don't waste time on slow nodes. Responders will
 /// try their best to either serve within that timeout or return an error immediately. (We need to