Neutron need to reduce number of network db calls during get_devices_details_list

Currently each Neutron agent will impose db calls to Neutron Server to query devices, port and networks when it get start up.

Take ml2 rpc.py method get_device_details for example:
It can be noticed that during this call:

it will get each port and then get each network that port associated within get_bound_port_context. Actually in prodcution env. Most ports are belonging to a much less number of networks. Maybe even one compute host with 500VM with single network.

As the netwok information is static information and was used to construct the device json. It's suggested to initially having these network prefetched. instead of query it each time within each port. This will reduce the db calls a lot.

with less db calls, it will benefit neutron server performance and database hit when VM number in large.