Purge Circulations: Better selection logic

Purge Circulations: Better selection logic

This attempts two things:

1 - Keep the number of circs the library wants based on the most recent
    circulations, regardless of user prefs and such.

    This is accomplished by changing the circ fetching query to use a
    window function in a subquery so we can filter on it easily.

    This also allowed removal of the purge position counter.

2 - Allow removal of circs that are between kept circs if they meet all
    other removal criteria.

    This is accomplished by using CONTINUE instead of EXIT in the loop.

Signed-off-by: Thomas Berezansky <tsbere@mvlc.org>