Layer 2 forwarding enhancements HLD (#379)

* Layer 2 Forwarding Enhancements FDS

The documents decribes the functional spec and high level design for Layer 2 Forwarding Enhancements.

* Layer 2 Forwarding Enhancements HLD

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

* Update .DS_Store

* Update .DS_Store

* Updated to change FDB flush as bulk flush in hardware

Changed FDB flush mechanism from individual FDB entry delete to Bulk flush in hardware.

* exclude bridge_port_id in fdb data structure

Updated to exclude bridge_port_id in Orchagent FDB data structure as it is not needed for bulk flush. We only need the FDB type.

* exclude bridge_port_id in fdb data structure

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

some minor corrections.

* Updated with inputs from community review

1. Added per port per VLAN fdb clear option.
2. Listed all the SAI attributes being used.
3. Added -w option in VLAN Range commands to indicate logging of warning messages.

* remove .DS_Store

* Update SONiC Layer 2 Forwarding Enhancements HLD.md

Updated default FDB aging time.