WIP: merge single-host spans in the UI

[codefromthecrypt]

Finally we can fix #1799 not that the UI code uses a tree internally

Consider you are using single-host spans (ex disabling join), with this change, the UI can merge them together as if they were in the same span. The new UI (Zipkin Lens) will likely handle this differently.. this is just a stop-gap.

Note: the UI currently re-parents the children of the server to the client span. I hypothesise most won't notice (as they don't l look at parentId in the UI only span Id). This is the cheapest way to support the feature, but we can do more if important. Do mention if it is.

cc @openzipkin/ui @openzipkin/core

Before:

After:

[codefromthecrypt]

will fix add tests etc after a few votes come in

[bogdandrutu]

@adriancole why not modify the client span-id to be server span-id?

[codefromthecrypt]

this is only internal to UI because javascript needs string key. the ids here are used for collapse feature and we need a primary ID string per row. could make that a concatenation as it wont matter. @drolando asked offline to have the "show ids" pane show both client and server ID. that is fine and will do.

[codefromthecrypt]

@tacigar probably better to do this in lens and not in the old UI. sound good?