Support multiple layers of nodes and edges in plexus

[everett980]

This is a tracking issue to encompass the work done across many PRs to achieve this goal.

Brief overview of functionality covered in first revision:

• Users must specify a single definition for calculating the size of nodes in the graph
• Users can specify one or more other layers of nodes that are sized to fit based on their corresponding measured node.
• Users can specify either HTML or SVG renderers for node layers.
• Users can specify multiple layers of SVG edge layers.
• Users can specify unique or shared edge markers for SVG edge layers.
• Jaeger uses new plexus API.

List of PRs by author:
(@tiffon in #426, #428, #422, #420, #419, #416, and #414)

[tiffon]

@everett980 Plexus supports measurable SVG node layers. So, the list item

Users can specify either HTML or SVG renders for non-measure-able node layers.

Can just be

• Users can specify either HTML or SVG renderers for node layers.

Also, what outstanding work is this ticket related to? Everything you've listed is done.

[everett980]

Updated description.

This issue was made as a tracking ticket for the changelog to connect multiple PRs to the effort, as the changelog entry for this work would've otherwise been unwieldy.