Remove ballast from genesis block

[nazar-pc]

We had a piece of data in the header, which worked acceptably well when segments were tiny. When we increased recorded history segment to 128MiB, suddenly we started getting issues with RPC that can't return the data, when returned JS will be slow to decode and moving ballast to extrinsics requires even more hacks since object mapping fails to allocate enough memory in WASM for such a big extrinsic.

In the end I decided to leverage the fact that in SCALE encoding size of following data (except in special cases) is known beforehand. So by appending some extra data after encoded data we can store whatever we want without affecting decoding. This is exact what I did here, adding essentially invisible pseudo-random data to the end of the encoding of genesis block.

The first commit is necessary, but it was also useful regardless, hence separate commit.

@dariolina I do not recall if we mention somewhere how we do this, but if we do, then we need to update older description to reflect this change. Ended up being quite elegant actually.

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[dariolina]

Smart! Added a sentence on how archiving is bootstraped to consensus spec here