Bug 15632: Koha::Patron::Messages - Add new classes

 Bug 15632: Koha::Patron::Messages - Add new classes

The following 4 CRUD subroutines in C4::Members:
- AddMessage
- DeleteMessage
- GetMessagesCount
- GetMessages

could be replaced with a new package based on Koha::Objets.
This patchset will add the 2 Koha::Patron::Message[s] classes, then use
it to replace the different calls to these subroutine.
It will slightly reduce the size of C4::Members

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>