Allow configuring LRU cache size in app.toml

Summary

Node operators should be able to configure LRU cache size based on their memory availability

Problem Definition

Storing instances in the LRU will have no effect on the results (still deterministic), but should lower execution time at the cost of increased memory usage. We cannot pick universal parameters for this, so we should allow node operators to set it.

Proposal

There seems to be an app.toml that the node operator can configure some subjective values like halt-height and min-gas-fee or such. Let's add one more value:

[wasm]
lru_size = 10

The default should be 0, which means disabled. This value must be parsed at start and is passed into the wasm.NewKeeper function. We currently have hardcoded the value to 3: https://github.com/cosmwasm/wasmd/blob/master/x/wasm/internal/keeper/keeper.go#L44

For Admin Use

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