"whitelisting" mechanism (list of files - stored in DB) - which
protecting us from downloading new files after upgrade/downgrade was
broken. And seems it became over-complicated with time.
I replacing it by 1 persistent flag inside downloader:
"prohibit_new_downloads.lock"
Erigon will turn downloader into this mode after
downloading/verification of first snapshots.
```
//Corner cases:
// - Erigon generated file X with hash H1. User upgraded Erigon. New version has preverified file X with hash H2. Must ignore H2 (don't send to Downloader)
// - Erigon "download once": means restart/upgrade/downgrade must not download files (and will be fast)
// - After "download once" - Erigon will produce and seed new files
```
------
`downloader --seedbox` is never "prohibit new downloads"
This PR adds support to store the transaction dependency (generated by
the block producer) in the block header for bor. This transaction
dependency will then be used by the parallel processor
([Block-STM](https://github.com/ledgerwatch/erigon/pull/7812/)).
I have created another
[PR](https://github.com/ledgerwatch/erigon-lib/pull/1064) in the
erigon-lib repo which adds the `IsParallelUniverse()` function.
System contract upgrades for Polygon are already handled by the
`BlockAlloc` logic and there's no need to duplicate it with the
`CalcuttaBlock` logic (there's no Calcutta in
https://github.com/maticnetwork/bor).
