Block relay improvements (take 2)

[nazar-pc]

This is exactly the same as #1883 except last commit. Incremental archiver seems to be an attempt to create something similar to SCALE encoding that is not quite SCALE encoding. There are simpler/nice ways to do that, but it is ultimately completely unnecessary as the last one-liner commit here achieves the same goal of never exceeding the size limit.

All credits for implementation here go to Rahul.

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1. Add support for returning multiple complete blocks, without going through the protocol. This is meant for scenarios where a node is syncing upon start
2. Enable block relay by default.
3. Clean ups

Fixes #1901

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