Instead of writing log errors to stderr, write them to a file in the state directory. When reporting a health check failure, include the path to the file and the last several lines.
Log files are named by date, and the last 7 days are kept.
Closes#106
Instead use the time at which the STH was observed (which for
FilesystemState is assumed to be the mtime of the STH file). This is
easier to reason about: we don't have to worry about logs lying about
the time; we don't have to take into account the delay between STH fetch
and healthcheck; we won't raise spurious health checks about logs with
MMDs longer than the healthcheck interval.
