Use a sanely sized canvas for the span mini-map

[tiffon]

Fix #256.

Optimize the span mini-map in the trace detail view to not create a huge canvas when dealing with large traces.

Time to draw the mini-map for a trace with around 10,000 spans:

Duration	Seconds	Ratio
Before	1.437 seconds
After	0.093 seconds	0.06

The main code change is to use sane limits for width and height when sizing the canvas.

[codecov]

Codecov Report

Merging #257 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
- Coverage    77.6%   77.59%   -0.01%     
==========================================
  Files         135      135              
  Lines        2934     2933       -1     
  Branches      608      608              
==========================================
- Hits         2277     2276       -1     
  Misses        520      520              
  Partials      137      137

Impacted Files	Coverage Δ
.../components/TracePage/SpanGraph/CanvasSpanGraph.js	100% <100%> (ø)	⬆️
...mponents/TracePage/SpanGraph/render-into-canvas.js	100% <100%> (ø)	⬆️

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