sync_pol: use the last policy update as the current policy

The controller must keep track of the current, authoritative policy for
the network. While there may be delay in actually implementing that
policy on the network, that delay should be minimized.

Prior to this commit the controller used the current policy for the
application it was running to determine the current, authoritative
policy for the network. But when the user specifies a non-zero
policy_queue_size, the application's current policy may differ greatly
from the one installed on the network, and that difference may persist
for quite some time. The result is that many packets sent to the
controller will be subject to a policy that is significantly different
from the network policy that forwarded them to the controller in the
first place.

This commit changes the bookkeeping of the current policy so that the
current policy is determined when the controller begins to install the
latest policy update on the network.
Merge pull request #337 from frenetic-lang/sync_pol

sync_pol: use the last policy update as the current policy