I also enforced some invariants in the signals code, fixing a couple
bugs. The signals code is more complex than I'd like, but hopefully
is working now.
I'd once thought about using 1 second resolution for signals and wrote
this map to match that. But I decided to match signals to the timestamps
used elsewhere instead. Match that for the days map also.
My main goal is to support creating indexes for signals as well as
recordings. An additional goal is to just shrink db.rs a bit; it's
gotten quite large.
