PURPOSE Simplify 'mail.tracking.value' model and code. Remove unnecessary fields and computation. Make code easier to handle and more batch-enabled. SPECIFICATIONS Rename 'field' to 'field_id' on tracking model. It better indicates it is a many2one and not a char field holding a field name for example. Task-3345979 (Mail: Simplify tracking model) Part-of: odoo/odoo#124182