Decouple elasticapm/opentracing

[axw]

Introduce an internal package which is used
instead of using context.WithValue/context.Value
directly. By default those functions are used,
but when apmot is imported, it overrides their
use with the opentracing context functions,
wrapping all transactions and spans such that
they implement the opentracing.Span API.

[codecov-io]

Codecov Report

Merging #235 into v2 will decrease coverage by 0.3%.
The diff coverage is 76.06%.

@@            Coverage Diff             @@
##               v2     #235      +/-   ##
==========================================
- Coverage   80.67%   80.37%   -0.31%     
==========================================
  Files          88       91       +3     
  Lines        5295     5396     +101     
==========================================
+ Hits         4272     4337      +65     
- Misses        816      850      +34     
- Partials      207      209       +2

Impacted Files	Coverage Δ
module/apmot/noop.go	100% <100%> (ø)
module/apmot/tracer.go	88.88% <100%> (+0.92%)	⬆️
gocontext.go	93.75% <100%> (ø)	⬆️
internal/apmcontext/context.go	100% <100%> (ø)
module/apmot/span.go	86.45% <100%> (-0.43%)	⬇️
module/apmot/wrapper.go	62.68% <62.68%> (ø)
module/apmot/context.go	86.95% <80%> (-13.05%)	⬇️
internal/fastjson/writer.go	68.83% <0%> (-2.6%)	⬇️
tracer.go	83.03% <0%> (-0.9%)	⬇️
model/marshal_fastjson.go	86.54% <0%> (-0.33%)	⬇️
... and 3 more

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