snapshots: reduce merge limit of blocks to 100K

[AskAlexSharov]

Reason:

• produce and seed snapshots earlier on chain tip. reduce depnedency on "good peers with history" at p2p-network.
  Some networks have no much archive peers, also ConsensusLayer clients are not-good(not-incentivised) at serving history.
• avoiding having too much files:
  more files(shards) - means "more metadata", "more lookups for non-indexed queries", "more dictionaries", "more bittorrent connections", ...
  less files - means small files will be removed after merge (no peers for this files).

ToDo:
[x] Recent 500K - merge up to 100K
[x] Older than 500K - merge up to 500K
[x] Start seeding 100k files
[x] Stop seeding 100k files after merge (right before delete)

In next PR:
[] Old version of Erigon must be able download recent hashes. To achieve it - at first start erigon will download preverified hashes .toml from s3 - if it's newer that what we have (build-in) - use it.