BUG/MINOR: filters: Reject id-less implicit instances of multi-instance classes

BUG/MINOR: filters: Reject id-less implicit instances of multi-instance classes

An id is mandatory when a filter class allows several instances per proxy,
and this was verified when a filter-config directive was parsed. But it was
not when an implicit instance was created by flt_add_implicit_instance()
called from another directive: for such a class, an instance without id
could not be distinguished from a second one.

Like for an explicit declaration, the implicit declaration is now reported
and ignored if no id is provided for such a class. All the in-tree
directives creating implicit instances of a multi-instance class (the
cache-store rules) already pass the cache name as id, so nothing changes for
existing configurations.

The filters documentation was updated accordingly.

No backport needed.