improve message numbering scheme to make better use
of "startrow" and "maxrows" functionalities provided by
POP clients written in C++, Java, and other popular
clients. Ultimately, it would be nice to be able to easily
query the server for the latest twenty messages, using
first a STAT and then querying for a certain number of
msgs. However, the current message numbering scheme
means that the latest messages may not have the
highest message number (in fact far from it). This
means clients connecting to hmailserver cannot optimize
their requests by calling for a STAT and then getting the
latest messages based off the previous call.