Paranoia nit: clean up _coins_busy_shuffling along with other data
Paranoia nit: clean up _coins_busy_shuffling along with other data

This is on thread exit anyway so it doesn't matter -- but it is good to
strongly enforce the invariant that _coins_busy_shuffling is empty
after thread exiti, always.

fyookball/electrum,fyookball/electrum,fyookball/electrum