Calling grpcbox_channel:stop() causes crash reports on active streams

[psalin]

grpcbox_channel:stop() uses gproc_pool:force_delete which will kill the processes and their children. This causes crash reports when there are active streams in the h2 connection. When dynamically opening and closing channels these crash reports can appear regularly. Since they do not mean an error in this case, an alternative, more silent way of shutting down the channel would be needed.