Make the baggage API fully functional in the API

[jkwatson]

and get rid of the BaggageManager that was needed previously.

This moves the Baggage implementation to the API, per open-telemetry/opentelemetry-specification#1103, which is now merged.

I could split this up, if desired, into 2 parts:

1. Move the implementation to the API, remove it from the SDK, but keep the BaggageManager in place.
2. Remove the BaggageManager entirely.

[codecov]

Codecov Report

Merging #1822 into master will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1822      +/-   ##
============================================
+ Coverage     85.47%   85.58%   +0.11%     
+ Complexity     1448     1434      -14     
============================================
  Files           177      174       -3     
  Lines          5589     5556      -33     
  Branches        580      579       -1     
============================================
- Hits           4777     4755      -22     
+ Misses          612      601      -11     
  Partials        200      200              

Impacted Files	Coverage Δ	Complexity Δ
.../src/main/java/io/opentelemetry/OpenTelemetry.java	97.91% <ø> (-0.24%)	20.00 <0.00> (-2.00)
.../opentelemetry/opentracingshim/BaseShimObject.java	100.00% <ø> (ø)	5.00 <0.00> (-1.00)
...in/java/io/opentelemetry/sdk/OpenTelemetrySdk.java	100.00% <ø> (ø)	2.00 <0.00> (-1.00)
...rc/main/java/io/opentelemetry/baggage/Baggage.java	100.00% <100.00%> (ø)	2.00 <2.00> (?)
...in/java/io/opentelemetry/baggage/BaggageUtils.java	100.00% <100.00%> (ø)	7.00 <0.00> (ø)
...ava/io/opentelemetry/baggage/ImmutableBaggage.java	96.49% <100.00%> (ø)	16.00 <4.00> (?)
...etry/baggage/propagation/W3CBaggagePropagator.java	93.18% <100.00%> (-0.44%)	15.00 <0.00> (ø)
...opentelemetry/opentracingshim/SpanContextShim.java	88.57% <100.00%> (ø)	9.00 <0.00> (ø)
...o/opentelemetry/opentracingshim/TelemetryInfo.java	100.00% <100.00%> (ø)	5.00 <1.00> (-1.00)
...va/io/opentelemetry/opentracingshim/TraceShim.java	100.00% <100.00%> (ø)	3.00 <0.00> (ø)
... and 7 more

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[anuraaga]

Mostly good, just one point about visibility - presume since we have an interface, we don't need to make the implementation of baggage public

[anuraaga]

🎉

[anuraaga]

Thanks! Can you rebase to get the build fix?

[carlosalberto]

Late review, thanks for this!