listbanned should return the ban duration and time remaining #90

From the BTC 22.0 relnotes:

The listbanned RPC now returns two new numeric fields: ban_duration and time_remaining. Respectively, these new fields indicate the duration of a ban and the time remaining until a ban expires, both in seconds.

This helps easily see if a ban is for the default or a custom duration, and when the ban will end.

time_remaining will be easy to add, but ban_duration is never stored internally, so that will require more changes.

time_remaining has been added to the dev branch