availability-recovery: bump chunk fetch threshold to 1MB for Polkadot and 4MB...

availability-recovery: bump chunk fetch threshold to 1MB for Polkadot and 4MB for Kusama + testnets (#4399)

Doing this change ensures that we minimize the CPU usage we spend in
reed-solomon by only doing the re-encoding into chunks if PoV size is
less than 4MB (which means all PoVs right now)
 
Based on susbystem benchmark results we concluded that it is safe to
bump this number higher. At worst case scenario the network pressure for
a backing group of 5 is around 25% of the network bandwidth in hw specs.

Assuming 6s block times (max_candidate_depth 3) and needed_approvals 30
the amount of bandwidth usage of a backing group used would hover above
`30 * 4 * 3 = 360MB` per relay chain block. Given a backing group of 5
that gives 72MB per block per validator -> 12 MB/s.

<details>
<summary>Reality check on Kusama PoV sizes (click for chart)</summary>
<br>
<img width="697" alt="Screenshot 2024-05-07 at 14 30 38"
src="https://github.com/paritytech/polkadot-sdk/assets/54316454/bfed32d4-8623-48b0-9ec0-8b95dd2a9d8c">
</details>

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Signed-off-by: Andrei Sandu <[email protected]>