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btcclient+btcjson: feeRate to BTC/kvB

defaultMaxFeeRate was set to 1e8 / 10(sat/kb) as a parameter.
But BTC/kvB is the expected value, so the units was wrong.
This commit updates defaultMaxFeeRate to BTC/kvB and sets it to 0.1,
which is the default value of Bitcoin Core.
This commit also updates the comment to reflect the change.

Because maxFeeRate sanity check has been added in
bitcoin core v27.0 or later,
sendrawtransaction cannot be executed without this change.