fix(python): prevent duplicate invocations of action callbacks due to ack delay
fix(python): prevent duplicate invocations of action callbacks due to ack delay

To handle disconnections between the client and server, the client maintains a queue of
unacknowledged actions and resends them until an acknowledgement message is received.  However,
nothing was done on the server to ignore these duplicate actions.

janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,google/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,seung-lab/neuroglancer