add rate limiting to outbound p2p msg sending

[antiochp]

Resolves #3553

We treat peers as abusive if inbound or outbound msg rates exceed 500/min.

This PR modifies this to only consider inbound msg rate.
We do not want to treat a peer as abusive if we are responsible for sending lots of p2p messages.

Add basic "rate limiting" to write_message() by inspecting the previous timestamp tracked by the tracker.
We ensure 150ms has elapsed based on previous sent message before attempting to send another.
In normal operation this is rarely the case. We occasionally send a ping and a block header close together and these are now separated by 150ms.
The scenario where this does come into play is during "fast sync" when requesting large numbers of full blocks from a small number of peers. Specifically "block archival sync" against archival nodes.
We no longer flood the peer connection with hundreds of block requests and these are now spaced out to avoid hitting the abusive peer limit on the receiving end.

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As part of this PR cleaned up the readonly usage of tracker.
The following fns have been removed, replaced with exposing a simple tracker() accessor -

• last_min_sent_bytes
• last_min_received_bytes
• last_min_message_counts

For example -

    let received = peer.tracker().received_bytes.read().count_per_min();

This makes the read locks more explicit and removes a case where we take multiple read locks in one go.

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Introduced elapsed_since_last_msg() to RateCounter for convenience.
This simplifies the implementation of the rate limiting logic.

[antiochp]

Going to merge.
Inbound "abusive" peer checking is unchanged. This only applies to outbound traffic, which we should not be flagging as abusive.