add memory-usage to stats-provider numbers

[warner]

It would be nice for a Tahoe node to measure its own memory usage (when possible: i.e. under linux with a readable /proc/self/statm) and report them in the stats-provider data. This would also cause them to be logged to the foolscap logs each time the stats-gatherer asks for them.

There is code to measure the memory usage in source:src/allmydata/test/check_memory.py