Restore listening connections when netstack s/r is enabled.

[copybara-service[bot]]

Restore listening connections when netstack s/r is enabled.

This CL restores the listening connections when netstack s/r is enabled. The changes include:

• New method as a workaround to replace the new routes and nics to the loaded stack after restore.
• New Restore() for transport layer protocols to restore the protocol level background workers.
• Adds afterLoad() method for fdbased processors.
• Adds a test to verify listening connection is restored after checkpointing with netstack s/r enabled.
• Few other changes to save restore fields to enable netstack s/r.