Add timing code to rb_newobj

Produces a rb_newobj.log file in the current directory.  Each
call to rb_newobj outputs a single line in that file with the number
of cycles that call took.
flush after print - seems to solve output issues
Reap all children
Don't start child GC if already running
Merge branch 'master' of github.com:Benabik/ruby-concurrent-gc

Conflicts:
	gc.c