update rpcclient for the method estimatefee

[jiangjinyuan]

Please update the method estimatefee in rpcclient! Actually, the Bitcoin ABC has removed nblocks parameter in estimatefee.
You can check this:
https://github.com/Bitcoin-ABC/bitcoin-abc/blob/dba3e93c015250c3a9739e4cce72a5aa260870ff/doc/release-notes/release-notes-0.19.1.md#bitcoin-abc-0191-release-notes

[zquestz]

I can look into this. =)

[mcroni]

any progress with this?