Get ms-since-epoch from native Date.

As nice as momentjs is, it is not free. Easy 15% improvement on the
benchmarks under benchmark/.
Log time as ISO8601 instead of seconds-since-epoch

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

As an additional benefit, the timestamps gain sub-second precision
which is very important for logging of highly concurrent apps.

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.