Sometimes calling code is only interested in the balance or UTXOs at a
single mixdepth. In these cases, it is wasteful to get the balance or
UTXOs at all mixdepths, only to throw away the returned information
about all but the single mixdepth of interest. Implement new methods in
BaseWallet to get the balance or UTXOs at a single mixdepth.
Also, correct an apparent oversight due to apparently misplaced
indentation: the maxheight parameter of get_balance_by_mixdepth was
ignored unless the include_disabled parameter was passed as False. It
appears that the intention was for include_disabled and maxheight to be
independent filters on the returned information.
Matt Whitlock