Add OpenTracing support to inbound and outbound pipelines.

[jmalloc]

This is approaching done-ness. I just wanted to push the PR now to get some feedback, specifically on the "event" names used in all of the pipeline stages.

As best I can tell, the only outstanding feature is logging application-logs (those made through the MessageContext) to the span, which should be fairly trivial.

[codecov-io]

Codecov Report

Merging #151 into master will decrease coverage by 0.16%. The diff coverage is 27.4%.

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
- Coverage   28.42%   28.26%   -0.17%     
==========================================
  Files          76       78       +2     
  Lines        2744     3064     +320     
==========================================
+ Hits          780      866      +86     
- Misses       1941     2174     +233     
- Partials       23       24       +1

Impacted Files	Coverage Δ
src/ax/endpoint/envelope.go	0% <ø> (ø)	:arrow_up:
src/axrmq/transport.go	0% <0%> (ø)	:arrow_up:
src/ax/endpoint/endpoint.go	0% <0%> (ø)	:arrow_up:
src/ax/saga/persistence/crud/persister.go	0% <0%> (ø)	:arrow_up:
src/ax/projection/consumer.go	0% <0%> (ø)	:arrow_up:
src/ax/saga/handler.go	0% <0%> (ø)	:arrow_up:
src/ax/endpoint/tracing.go	0% <0%> (ø)
src/axrmq/marshaling.go	0% <0%> (ø)	:arrow_up:
src/ax/endpoint/receiver.go	0% <0%> (ø)	:arrow_up:
src/ax/endpoint/transport.go	38.46% <0%> (-61.54%)	:arrow_down:
... and 12 more

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[jmalloc]

Ah, I am also yet to add tracing through the saga subsystem.

[jmalloc]

This PR is now ready for a second round of review guys. All of the saga logging and appication-layer logging has been added now.

[danilvpetrov]

I can't see anything requiring changes.