Major rewrite of the core code. Now parse_date_and_author returns a
time (i.e., seconds since the epoch) and author, rather than an
textual date and author. The whole structure of what we store has been
changed. No more building keys from concatenated strings (i.e., D.A.M.
keys), instead we use multi-level hashes. This change should reduce
memory use.

Previously, $Max_Checkin_Duration had an effective granularity of 60
seconds, due to the dropping of seconds from dates in
parse_date_and_author. Because parse_date_and_author now groks dates
to the second, it would now be okay to set $Max_Checkin_Duration to ten
seconds, whereas previously that would be identical to setting it to
zero.

(Melissa, I made some mods to this patch, first to get it to apply,
then to get it to run.  The upshot is: everything is untabified now,
hunk #7 was applied by hand [this was from patch #15], and since
apparently can't do "my (undef,...)" on line 304, I changed it to "my
($ignore,...)"  instead.  Maybe the original way is legal in a higher
version of Perl or something?  -kff)