client/{dcr,btc}: maxOrder loop needs to be a binary search

[buck54321]

With the new trade limits implementation, lot sizes can be thousands of times smaller than what we're used to dealing with. Our current maxOrder implementation does some rough estimation, and then zones in on the exact max order by looping with a lots-- until it finds the value. This is a bad idea if lot sizes are really small. Instead, we should do a binary search.

dcrdex/client/asset/btc/btc.go

Lines 1874 to 1885 in 6a12553

	for lots > 0 {
	est, _, _, err := btc.estimateSwap(lots, lotSize, feeSuggestion, maxFeeRate,
	utxos, true, 1.0)
	// The only failure mode of estimateSwap -> btc.fund is when there is
	// not enough funds, so if an error is encountered, count down the lots
	// and repeat until we have enough.
	if err != nil {
	lots--
	continue
	}
	return utxos, est, nil
	}