Plot server should gracefully handle endpoint clients vanishing

[PeterC-DLS]

Killing a benchmarking script early seems to disrupt the plot server. It does not respond to a new script running after that. Maybe it is waiting for the old http requester to respond...

To reproduce,

cd server
python davidia/demos/benchmark.py -t add_data 5 10240 64 128
# ctrl-c to kill script once it has ran for 2s
python davidia/demos/benchmark.py -t heatmap
# should see a heatmap in plot_0