Remove for_each_thread

This patch removes the for_each_thread function, changing the callers
to use 'foreach' loops instead.  In general I think loops with
iterators should be preferred over callback-based approaches -- they
are easier to read and often result in less source code as well.  For
example, in this patch a helper function is inlined into its sole
caller.

Approved-By: Simon Marchi <simon.marchi@efficios.com>