MINOR: activity: make "show profiling" also dump the memoery usage

MINOR: activity: make "show profiling" also dump the memoery usage

Now the memory usage stats are dumped. They are first sorted by total
alloc+free so that the first ones are always the most relevant, and
that most symmetric alloc/free pairs appear next to each other. This
way it becomes convenient to only show a small part of them such as:

    show profiling memory 20

It's worth noting that the sorting is performed upon each call to the
iohandler so it is technically possible that an entry could appear
twice or be dropped if the ordering changes between two calls. In
practice it is not an issue but it's worth being mentioned.