veyron/runtimes/google/ipc: cache network interfaces
veyron/runtimes/google/ipc: cache network interfaces

Cache available network interfaces on the device, since it is not
that cheap. (In my local machine, this saves about 1ms per RPC call.)

Will add monitoring the network interface changes once it's ready.

Change-Id: Iff75c67bdfd7eab7582ffa2b7a93d5485937269f

vanadium/go.lib