add t/stress/threads.t for GH #880 and GH #875
Move PObj_custom_mark_SET to the end of Task.init
[GH #880/#875] Apparently fixed the GC thread bugs

Also block the sweep phase from proxied interps. Previously only the mark.
Update t/stress/threads.t docs
fix threads: do not mark parent_interp
Fix -DTHREAD_DEBUG PARROT_GC_ASSERT_INTERP assertion in gc_gms for PMCNULL

This caused t/stress/threads.t test 2 to fail.