[Lib] Raised the gap_limit_for_change to 20. See #1277 as to why
[Lib] Raised the gap_limit_for_change to 20. See #1277 as to why

This way CashShuffle can live comfortably since it really needs a gap
limit for change of at least 12.

fyookball/electrum,fyookball/electrum,fyookball/electrum