Add "small message" net fallback

After our latest performance tests, it became clear that Express performs poorly for small messages. But we already knew that, Express wasn't made to send small messages - no big surprise.

Well, based on our current per-key storage metrics, people are sending tons very small messages using Express. Unfortunately with messages this small, their experience will be exclusively worse than if they had just used a net message.

The performance tests highlighted another important thing: Express is only worse for messages that require 3 or fewer net messages.

This is interesting. If we look at both the usage metrics and the performance results, we find a cheap way to improve Express.

I don't necessarily want to make a pure-net data sender, but we could pretty easily cover the situation where a message could fit into a single net message.

That's what this PR does; it redirects small message payloads through the net library instead of the Express API, while still behaving the same way (calling receivers, respecting preDl receivers handling proofs, etc.).

I want to take this opportunity to refactor a few things and slim down the code, so the diff will be large but the functional changes will be few.

I also need to decide what to use as the id for these "small" messages in the callback chain. (some callbacks want the item ID). I don't want to introduce a regression, but I also don't want to write any worse code...

CFC-Servers / gm_express

Add "small message" net fallback #36