Remove two sources of truth for default trie cache size

[matklad]

However, the overall code does feel a bit "bolted on". I would like to fix the following things in the follow up:

• make sure that default and overriden cache capacity is handled in the same way. It doesn't make sense that the former is hard-coded, but the latter is overridable

• Cleanup defaults in StoreConfig, it feels messy that the source of truth are awkward functions which exists solely for serde

• Replace TrieFactory with just TrieConfig -- we don't need doer object here, just a bag of values

• revisit the format of the config,

  "trie_cache_capacities": [
    [
      {
        "version": 1,
        "shard_id": 2
      },
      2000000
    ]
  ]
  

  doesn't look like a good config format, something like { "1:2": "2000000" } would'd be much user friendlier.

Originally posted by @matklad in #7022 (comment)

[matklad]

cc @Longarithm

[matklad]

cc @jakmeier I think this will be closed by one of your PRs

[jakmeier]

Solved by #7566 + #7578