connectivity multi bar charts: collect skeleton IDs in a safer way.
As it was the creation of the bar chart could fail if there weren't
any skeletons with one synapse in common with the active or selection
table skeletons.
More compact json for heavily used requests
like the list of nodes for the SVGOverlay (saves 5 to 10%)
and the list of nodes and properties of a skeleton for the 3d Viewer (saves 10%)