Overhaul memory management README.

The README was written as a "historical account", and that style
hasn't aged particularly well.  Rephrase it to describe the current
situation, instead of having various version specific comments.

This also updates the description of how allocated chunks are
associated with their corresponding context, the method of which has
changed in the preceding commit.

Author: Andres Freund
Discussion: https://postgr.es/m/20170228074420.aazv4iw6k562mnxg@alap3.anarazel.de