bug: Genesis imports degradation

[jhernandezb]

During testing and improving of genesis imports/exports I have identified a bottleneck in how genesis imports of contract state is happening specially when there is half million contract instances being imported

Specifically the import of historicalEntries creating unnecessary amount of iterators creating a lot of cpu usage and contention

wasmd/x/wasm/keeper/keeper.go

Lines 803 to 807 in b2294ae

	// find last element position
	var pos uint64
	prefixStore := prefix.NewStore(runtime.KVStoreAdapter(store), types.GetContractCodeHistoryElementPrefix(contractAddr))
	iter := prefixStore.ReverseIterator(nil, nil)
	defer iter.Close()

the appendToContractHistory can avoid entirely the creation of unnecessary iterators if we assume the entire history has been passed for insertion (there is no pre-existing position as it is the case for a genesis import)

I've ran a patched version jhernandezb@0c31fbc

Let me know if it is ok to submit similar patch

[pinosu]

Hey @jhernandezb , thanks for raising this issue.
Sounds good! Feel free to open a PR.