Wallet persists utxo metadata; currently only
contains a field 'disabled' indexed by utxo.
User can switch this on or off (enabled) via
wallet-tool 'freeze' method.
Disabled utxos will not be used in coin
selection in any transaction.
Wallet still displays all utxo balances in
display method (and in GUI).
Add tests of disabling to test_utxomanager
Add Coins tab to Qt, with freeze/unfreeze feature.
Coins tab shows updated utxo info txid:n, amt, address and
enabled/disabled, which can be toggled from right click menu.