[3.0.x] (bug #4263) fix the edition of items with repeatable subfields

[3.0.x] (bug #4263) fix the edition of items with repeatable subfields

The subfield management in item level is broken, fields are concatenated in one field, and if the librarian edit it, the values are not selected.
This big patch fix three things:
 1) saving fields that are stocked in SQL(using koha2marc mapping) are now well cut and separated in _REAL_ subfields
 2) loading records with repeatable subfields are now well returned
 3) Editing items with repeatable fields works well

Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>