Bug 25690: (QA follow-up) PROCESSED -> PROCESSING for consistency

Bug 25690: (QA follow-up) PROCESSED -> PROCESSING for consistency

PROCESSED gave the apearance that the item processing had been completed
whereas in reading the code it appears to actually signify that the item
is awaiting/in proessing state.

This patch updates the variable to be PROCESSING consistently throughout
the codebase.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>