config.mempool_fees is now [] if server claims mempool is ~empty,
and None if no valid histogram has been received from server.
(previously it used to be [] in both cases)
The few other cases that used SimpleConfig.get_instance() now
either get passed a config instance, or they try to get a reference
to something else that has a reference to a config.
(see lnsweep, qt/qrcodewidget, qt/qrtextedit)
