WARNING: This interface is unstable and may disappear or change!
Estimates the approximate fee per kilobyte needed for a transaction to begin
confirmation within nblocks blocks if possible and return the number of blocks
for which the estimate is valid.
1. nblocks (numeric)
"feerate" : x.x, (numeric) estimate fee-per-kilobyte (in BTC)
"blocks" : n (numeric) block number where estimate was found
A negative value is returned if not enough transactions and blocks
have been observed to make an estimate for any number of blocks.
However it will not return a value below the mempool reject fee.
> bitcoin-cli estimatesmartfee 6