Cleanup cache configuration code

[jakmeier]

Cache configuration is not ideal today. The format for cache capacities is verbose JSON, some field values are hard-coded without good reason, and the config code probably has too many structs (StoreConfig, TrieConfig, ShardCacheConfig).

We should carefully consider how the user-facing configuration should look like in the next step. And try to generally cleanup the code a bit more.

Already done: #7566

Already in progress: #7578

[jakmeier]

Done with #7578