Bug 22614: Migrate original request

Bug 22614: Migrate original request

This patch changes an active request's backend and initiates the
migration on that request, rather than creating another and migrating
that.

The majority of the work to accomplish this bug is being carried out in
the backends, since Koha delegates migrations to them.

Signed-off-by: Barry Cannon <bc@interleaf.ie>
Signed-off-by: Assumpta Byrne <abyrne@ait.ie>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>