Log time as ISO8601 instead of ms-since-epoch

Make log messages more portable by recording log line timestamps in
ISO8601 format instead of milliseconds-since-epoch.

Overhead is minimal and Date's toJSON() does all the hard work, but it
is comparable to using momentjs to convert the timestamp to seconds.
Don't re-calculate obviously static filenames

Filenames that are enclosed in [] are entirely static and don't need
to be re-interpretted for every log message.

In cases where simple data is being logged to a static file, the time
formatting used for the filename can account for 50% or more of the
time spent in a log() call.