This enables users to specify an external API for fee estimation.
The API is expected to return fees in the JSON format:
`{
fee_by_block_target: {
a: x,
b: y,
...
c: z
}
}`
where a, b, c are block targets and x, y, z are fees in sat/kb.
Note that a, b, c need not be contiguous.
In this commit, we modify our FeeEstimator interface to return an
estimated fee rate in sat/kw. Recently, due to low fees on the network,
users have been experiencing failures broadcasting transactions due to
not meeting specific fee requirements. This was happening more often
than not, as the estimated fee returned by backend nodes (bitcoind and
btcd) only takes into account vbytes, rather than weight. The fees
returned are also expressed in sat/kb, so we must take care that we do
not lose precision while converting to sat/kw. In the event that this
happens, a fee floor of 253 sat/kw has been added. This fee rate
originates from bitcoind rounding up the conversion from weight to
vbytes.
