Add method for setting the start date of a volume
Debug new start_date method in pulldb
Add updating volume start dates to pullck, and make both this new check
and the existing issue updates flag controlled.
Refactor database to use DATE type for start_date rather than TIMESTAMP
due to problems with timezone handling.  All entries are dates anyway,
so no need to store the time part which is always midnight...

Terminate missing issue check when get to the start of the oldest pulled
volume.  Saves having to download thousands of issues of history for a
title that is marked to start recently.