Convert CorrelationContext entry keys and values to simple Strings

[jkwatson]

Resolves #1334

[codecov]

Codecov Report

Merging #1363 into master will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1363      +/-   ##
============================================
- Coverage     86.89%   86.86%   -0.03%     
+ Complexity     1245     1241       -4     
============================================
  Files           159      157       -2     
  Lines          4791     4788       -3     
  Branches        431      432       +1     
============================================
- Hits           4163     4159       -4     
- Misses          467      468       +1     
  Partials        161      161              

Impacted Files	Coverage Δ	Complexity Δ
...ationcontext/DefaultCorrelationContextManager.java	56.25% <ø> (ø)	6.00 <0.00> (ø)
...ry/correlationcontext/EmptyCorrelationContext.java	75.00% <ø> (ø)	3.00 <0.00> (ø)
...ava/io/opentelemetry/correlationcontext/Entry.java	100.00% <100.00%> (ø)	10.00 <8.00> (+7.00)
...opentelemetry/opentracingshim/SpanContextShim.java	88.88% <100.00%> (+2.40%)	10.00 <1.00> (ø)
.../sdk/correlationcontext/CorrelationContextSdk.java	92.15% <100.00%> (ø)	15.00 <7.00> (ø)
...telemetry/sdk/trace/export/BatchSpanProcessor.java	94.59% <0.00%> (-1.36%)	8.00% <0.00%> (ø%)

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[carlosalberto]

Pretty sweet change, LGTM.

[carlosalberto]

Ping @bogdandrutu