Add first pkgdb conglomerators.

These three are all pretty similar - they group acl.update messages in three
different ways (if the package is the same, if the user is the same, if the
agent is the same).  We can add a lot more of these for pkgdb (for new
branches, new packages, etc) which will cut down on fmn spam.. but I just want
to submit this and see how it plays before doing more.  This seems like the
main use-case (acl.updates, that is).

Tests included and passing.