Change the sampling API to take a parent Context, rather than SpanContext

[jkwatson]

See this recent spec update: open-telemetry/opentelemetry-specification#881

[codecov]

Codecov Report

Merging #1840 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #1840   +/-   ##
=========================================
  Coverage     85.51%   85.51%           
  Complexity     1442     1442           
=========================================
  Files           177      177           
  Lines          5571     5572    +1     
  Branches        578      578           
=========================================
+ Hits           4764     4765    +1     
  Misses          606      606           
  Partials        201      201           

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/opentelemetry/sdk/trace/Sampler.java	100.00% <ø> (ø)	0.00 <0.00> (ø)
...ava/io/opentelemetry/sdk/trace/SpanBuilderSdk.java	96.80% <ø> (ø)	31.00 <0.00> (ø)
...ions/trace/jaeger/sampler/JaegerRemoteSampler.java	74.50% <ø> (ø)	9.00 <0.00> (ø)
...ions/trace/jaeger/sampler/PerOperationSampler.java	75.00% <ø> (ø)	4.00 <0.00> (ø)
...main/java/io/opentelemetry/sdk/trace/Samplers.java	77.67% <100.00%> (+0.20%)	14.00 <0.00> (ø)
...ions/trace/jaeger/sampler/RateLimitingSampler.java	69.56% <100.00%> (ø)	7.00 <3.00> (ø)

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[anuraaga]

Dunno why this looks like dejavu

[lzchen]

@jkwatson
You guys are so fast!