Before this change, when using an all create method with one of the
upstreams being read only, if there was an existing file on the read
only remote, it was impossible to update it.
This change detects that situation and creates the file on a
read/write upstream. This file will shadow the file on the read/only
upstream. If it is deleted the read only upstream file will be visible
again.
Fixes#4929
Before this change writing with the all policy deadlocked while
uploading.
This change fixes the problem by fixing the multi reader, closing the
pipes at the correct time with the correct error. This is factored
into a new function as it was used twice.
This patch also adds a new test which tests the all policies.
