save live plots into internal datastructure

fd606522 introduces the saving of live plots onto a list attached to a runner.

Instead of modifying the runner itself, we could instead store a mapping adaptive.active_plots that keeps the tasks around. This would be populated by adaptive.live_plot, which could take an optional parameter, name, that would be a key into this mapping (we maintain a pool of names plot_n for when name is not given explicitly). If a second plot gets created with the same name, then the updating task for the previous plot of the same name gets stopped and removed from the mapping.

This would prevent the "I executed the plotting cell twice and now can't stop the plots" problem. If a name was given, then the previous updating tasks are stopped automatically, otherwise the user has to manually find the plots in adaptive.active_plots and stop them.