Improve validators_buffer behavior in collator protocol

[rphmeier]

The new connection management for collator->validator connections has some issues. It occasionally disconnects from validators right before receiving the next collation and attempting to reconnect, triggering an underlying network issue.

#1499

We are probably too aggressive in resetting interest in collations. Validators will often not request a collation until its parent is backed, which can take some time.

It may be better to eagerly disconnect from validators we have sent collations to as soon as we provide it as well.
Alternatively, we might combine a two-pronged approach:

1. only disconnect validators 3 seconds after a new leaf
2. introduce a CollatorProtocolMessage::CollatorHeadsUp(RelayParent) to pre-connect to validators when the collator begins authoring.

[slumber]

Regardless whether validator requests collation its interest will be discarded after timeout:

polkadot-sdk/polkadot/node/network/collator-protocol/src/collator_side/validators_buffer.rs

Lines 221 to 222 in 2daa471

	/// A timeout for resetting validators' interests in collations.
	pub const RESET_INTEREST_TIMEOUT: Duration = Duration::from_secs(6);

Then, when RECONNECT_TIMEOUT secs pass (or if we issue another connection request because of new collation built), the connection-set will be updated, validators whose interest was reset will be disconnected.

polkadot-sdk/polkadot/node/network/collator-protocol/src/collator_side/mod.rs

Lines 96 to 102 in 2daa471

	/// Ensure that collator issues a connection request at least once every this many seconds.
	/// Usually it's done when advertising new collation. However, if the core stays occupied or
	/// it's not our turn to produce a candidate, it's important to disconnect from previous
	/// peers.
	///
	/// Validators are obtained from [`ValidatorGroupsBuffer::validators_to_connect`].
	const RECONNECT_TIMEOUT: Duration = Duration::from_secs(12);

These values were chosen for sync backing, so we should just tune it.