Need to clear various group storage/state on fabric removal

[bzbarsky-apple]

Problem

When a fabric is removed, we need to clear various state that is associated with the fabric index, because the fabric index can get reused for a different fabric.

This includes at least the following for groups:

• Group peer message counters.
• Group/endpoint associations from the groups cluster.
• Group keys associated with the fabric.
• Group id/key associations for the fabric.

Proposed Solution

Remove the various data when the fabric index becomes stale.