add ability to collapse spans below threshold

[roippi]

(moved from jaegertracing/jaeger#1724)

Requirement

When I view a trace, I would like the option to collapse all parent spans with duration less than some configurable threshold.

Problem

The default trace view expands all spans by default.

jaegertracing/jaeger#1552 proposed dropping spans below some threshold. This is not what I want - I do like that these small spans are there; they just take up a lot of vertical space and make the default trace view very tall. I'd like to see them only when I drill in.

#160 is an umbrella issue for heuristics that could possibly encompass this (but did not mention this particular method).

I can of course manually collapse spans to get a view that I'd like, but this is time consuming and requires some training of engineers to help them get the most out of the tool.

Proposal

A query param to be provided for trace view to switch on this behavior, eg ?collapseBelow=100ms. Exact syntax to be bikeshedded :)

[roippi]

possibly closes #389 as well.