Title: Enforce prevention of multiple instances of a locality in the same priority

Description:
PR #20574 refined the LocalityLbEndpoints structure to be defined as:

// A group of endpoints belonging to a Locality.
// One can have multiple LocalityLbEndpoints for a locality, but only if
// they have different priorities.

but Envoy allows to have multiple LocalityLbEndpoints for a locality if the different groups have different weights or different priorities.

Envoy should be more restrictive, and confirm to the revised comment.

cc @markdroth