Convey timestamps in written changesets

The Publisher/PublisherService now generate paths which include the
WikiPage timestamp (last-modified date). The path format now is:
YYYY/MM/DD/HH/TTTTTTTTTTTTTTTTTTTT-SSSSSS.{added,removed}.nt.gz

With Y, M, D, H standing for the year, month, day and hour components, T
representing a digit of the timestamp value (milliseconds from epoch)
and S being a sequence number to avoid possible collisions.

This update may break compatibility with dbpintegrator, as filenames
changed (sequence number now the 6th component when splitting by "-").

Also, we'll probably see an increase in disk I/O. Maybe we can figure
out an elegant way of batching writes while retaining the ability to
identify written changesets for any given timestamp in the JSONCache
database.