Soft-reset the module between handler calls

[leoyvens]

After #702 we re-instantiate the WASM runtime between each call, to free memory and ensure no state is carried over. This works well, but it can cost us a couple milliseconds on each call. A soft reset of the module, which means resetting memory contents and size, and also global state, would likely be more performant. This was attempted in this branch with no success. Parity needs to do similar things with their runtime, so we may be able to cooperate with them when revisiting this in the future.

[leoyvens]

It seems that Substrate figured this out, but it added some restrictions and seems complicated paritytech/substrate#3011

[Jannis]

@leoyvens Has this changed with switching to wasmtime or is this still something we might want to do?

[leoyvens]

It's still an interesting optimization in theory, but I don't think this will be worth pursuing.