Manually track memory usage

Summary:
memory_get_usage() does not return useful results at it seems like the
bulk of the memory is used outside of the php engine (e.g. in
resources).

RSS usage can for instance be tracked using the following snippet:

`ps --pid $pid --no-headers -orss`;

The current accumulation method is in no way accurate and will probably
require some tuning to be on the safe side.

Differential Revision: https://git.kolab.org/D1237