Estimates the approximate fee per kilobyte needed for a transaction to begin
confirmation within nblocks blocks. Uses virtual transaction size of transaction
as defined in BIP 141 (witness data is discounted).
1. nblocks (numeric, required)
n (numeric) estimated fee-per-kilobyte
A negative value is returned if not enough transactions and blocks
have been observed to make an estimate.
-1 is always returned for nblocks == 1 as it is impossible to calculate
a fee that is high enough to get reliably included in the next block.
> bitcoin-cli estimatefee 6