[FIX] [IMP] mail: refactored code of message_auto_subscribe.

This new code allows faster computation, because it avoids
- searching in a many2many relationship that is very costly in production
databases, because it is replaced by an 'id in [ids]' equivalent leaf
that can be huge.
- browsing in a create/write (should also lessen the number of issues
about read access rights in create)

A new optionnal parameter is given to the method, that are the
modified values. If this is not given (compatibility-mode) the
records are browsed to fetch the value.
[FIX] mail: auto_subscribe: domain for fetching followers was wrong, forgot an AND

[FIX] [IMP] mail: refactored code of message_auto_subscribe.

This new code allows faster computation, because it avoids
- searching in a many2many relationship that is very costly in production
databases, because it is replaced by an 'id in [ids]' equivalent leaf
that can be huge.
- browsing in a create/write (should also lessen the number of issues
about read access rights in create)

A new optionnal parameter is given to the method, that are the
modified values. If this is not given (compatibility-mode) the
records are browsed to fetch the value.