Get seconds-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.
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.