Added some logic to prevent change address gaps
Added some logic to prevent change address gaps

Change addresses would stay "reserved" for shufflers that didn't end up
using them (due to getting lucky and being the minimum amount guy), and
thus would create gaps in the change addresses until next time
plugin/app was restarted, where they would become eligible again to be
used as output addresses.

This has now been fixed and no gaps should appear in the change
addresses on continuous app run.  All addresses get used eventually if
they are new (0 tx)!

fyookball/electrum,fyookball/electrum,fyookball/electrum