zcash: fix tx rate handling

[buck54321]

ZCash fee handling is wrong. ZCash currently uses a standard fee of . 0001 per tx. We're still using it as a rate per byte. We're also using medianFeesTheHardWay, which scans full blocks. This is going to be expensive until the spam attack is fixed. We should simplify it and use the constant. We'll have to assume different units in a number of places. For example, what we return from the fee_rate API or assign to Match.{FeeRateBase,FeeRateQuote} is expected to be a rate/size. I think for zcash we have to interpret it as a rate/tx and eventually a rate/(input_count + output_count) once https://github.com/zcash/zips/blob/main/zip-0317.rst is in.

[chappjc]

Good desc, and reasonable way to handle it for now. But is this a dup of https://github.com/decred/dcrdex/issues/2239?

[buck54321]

Oh. I completely forgot I opened that.

[buck54321]

Now handled in #2553