1. Increase default MAC learn limit from 1M to 8M entries.
2. Check MAC learn limit in MAC learning update path.
3. Allow disable of want_l2_macs_events to set MAC learn limit
4. Other minor cleanups
Change-Id: I62438440937b5fa455e16f4a2e4d910277753395 Signed-off-by: John Lo <loj@cisco.com>