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)
SimpleConfig claims to handle configuration options in priority
command line, user config, system config, which makes sense.
In fact it appears it used priority command line, system config,
user config.
Also, from the priority ordering, it would seem correct that a
value should be unmodifiable if and only if it's set on the command
line. Previously anything in the system config would be unmodifiable.
This patch fixes these and cleans the code up a bit. I noticed this
whilst attempting to unify the 'auto_cycle' setting.
Fixup tests accordingly.
SomberNight
Dzhelil Rufat
Janus
Justin Turner Arthur
ThomasV
Dmitry Sorokin
Thomas König
Neil Booth
Chris Glass