billing: Fix date handling for json and yaml output

The raw date format in billing files lacks a year. The billing utility
has worked around that by prefixing the output with the full date. For
JSON and YAML output, this leads to a faulty timestamps like this:

    Jan 1, 2200: 01.01 00:00:00

This patch resolves the problem by converting it to an ISO8601 compliant
format (the preferred format for JSON and YAML).

The raw output is also adjusted such that the default format is prefixed
by the year rather than the full date.

JSON and YAML output formats have beeen adjusted to skip unrecognized
records. For YAML, the raw billing line is included as a comment.

Finally, the patch fixes an issue with outputing JSON from multiple days.

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7357/