Caching of etherscan.io transactions and/or block #->timestamp mappings

[JakubNer]

https://github.com/JakubNer/overhide-ethereum/blob/77e54ebefdd19d23425d1b401308a98ee6f99cdb/main/js/glue/get-transactions.js#L13

At the moment all transactions are pulled down for the 'to' address from etherscan.io, processed, discarded.

Considering that most checks are for a limited number of 'to' addresses (broker & services are the 'to' addresses checking subscribers) and for some limited time frame ('since' parameter specified); it's likely worthwhile to cache.

If we don't want to cache specific transaction sets, we might want to cache Ethereum block #s to timestamps such that when asked for transactions 'since' some date, we can limit the etherscan.io API calls to start from a certain block #.