Add a simple eth_call cache

[Nana-EC]

Problem

In some cases DApps may poll the relay eth_call endpoint quite often even when there are no contract changes.
In the current iteration prior to HIP 584 completion is may be beneficial to consider a cache to speed up responses

Solution

Add an appropriate cache

• Check the Mirror Node api/v1/contracts/{contractIdOrAddress} for the timestamp of the last change

• Cache the timestamp

• On future calls query the endpoint. If the timestamp returned is greater than the cached value then it's worth making a full state call

• Also add a new metric for gas used by an endpoint. Start with a proxy of 0.8 * gasLimit provided

Alternatives

No response

[steven-sheehy]

api/v1/contracts/{contractIdOrAddress} timestamp would only update for CUD operations