The output stage can now write to size-limited, rotating WARC files
using a user-specified pattern, so that output files are always
unique.

Detect write errors (both on the database and to the WARC output) and
abort with an error message.

Also fix a bunch of harmless lint warnings.

The WARC-Date fields now are UTC times in proper ISO-8601 format. This
makes pywb and other tools happy.

Now generates well-formed, indexable WARC files.