Attempt to fix "Unable to set thread priority" bug

Issue 8960 http://d.puremagic.com/issues/show_bug.cgi?id=8960

In std.parallelism code which sets thread priority comes after starting it. If thread execution is short, thread may terminate before setting priority. This pull makes Thread.start() function to set priorities (if any) before launching thread.

Trivial fix to std.parallelism will be provided after this pull.