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. Uses virtual transaction size as defined
in BIP 141 (witness data is discounted).
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