Multi-session support for plots

[SimonHeybrock]

TL;DR seems to be:

• hv.streams.Pipe must be per-session
• hv.DynamicMap must be per-session
• We need one periodic-callback per session. Pushing to all pipes from a single callback does not work. Each session's callback pushes to that session's pipes.

If the above is true (I am still not certain that this is the case!) then we need to restructure a number of things. First, Plotter subclasses do substantial work. Currently they are called by the DynamicMap, which would now become per-session. Currently we have:

periodic_callback -> Orchestrator -> DataService -> Pipe -> DynamicMap (uses Plotter)

We probably want something like:

background_thread -> Orchestrator -> DataService -> Plotter -> PlotDataService
# per session
periodic_callback -> PlotDataService.get_data -> Pipe -> DynamicMap

These are still early ideas.