Read vif port information in bulk

Read vif port information in bulk

During startup, the agent was making many calls per port
to read information about the current VLAN, external ID, etc.
This resulted in hundreds of calls just to read information about
a relatively small number of ports.

This patch addresses that by converting a few key functions to
lookup information for all of the ports at once.

Performance improvement on dev laptop for 250 ports from agent
start to port ACTIVE status:
   before: 1m21s
   after: 1m06s

Closes-Bug: #1460233
Change-Id: Ic80c85a07fee3e5651dc19819c6cebdc2048dda7