[dmd-concurrency] draft 2

[Andrei Alexandrescu]

I operated a few small changes to the intro, finished section "A Brief
History of Data Sharing" and worked into the discussion of shared and
non-shared globals.

See http://erdani.com/d/fragment.preview.pdf

I still have in mind that the flagship approach is message passing, but
in order to explain the "no-share" approach I also need to explain under
what conditions sharing is actually possible.

Feedback is welcome. Thanks!


Andrei