Add a master switch so that the sync/async mode can be configured.
Example usage of the switch:
1. Configure mode while building an image
`make ENABLE_SYNCHRONOUS_MODE=y <target>`
2. Configure when the device is running
Change CONFIG_DB with `sonic-cfggen -a '{"DEVICE_METADATA":{"localhost": {"synchronous_mode": "enable"}}}' --write-to-db`
Restart swss with `systemctl restart swss`
