Bug 18837: Introduce delegation to backends for unmediated workflows

Bug 18837: Introduce delegation to backends for unmediated workflows

This patch adds support to Illrequest.pm to allow delegation of
unmediated requests to happen in backends that support them.

It is a recreation of
https://bugs.koha-community.org/bugzilla3/attachment.cgi?id=66629&action=diff
which had diverged sufficiently as to make it impossible to apply

Signed-off-by: Stephen Graham <s.graham4@herts.ac.uk>
Signed-off-by: David Peacock <d.m.peacock@herts.ac.uk>
Signed-off-by: Jayne Maisey <j.maisey@herts.ac.uk>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>