blockchain.estimatefee should be cached (instead of doing a daemon RPC every time)

[SomberNight]

I think it's expensive to call bitcoind in general, so it might be beneficial to eliminate at least the low hanging fruit:
specifically it should be relatively easy to cache "estimatefee" coming from bitcoind, e.g. in a time based cache for 1 minute, and return those values when requested from clients through blockchain.estimatefee from the cache without querying bitcoind.