Upgrade jaeger-lib to 2.2 and unpin Prom client

[yurishkuro]

Resolves #433

Currently uses private branch of jaeger-lib, pending jaeger-lib 2.2 release

TODO:

• merge Upgrade Prometheus client to 1.1 and go-kit to 0.9 jaeger-lib#78
• release jaeger-lib 2.2
• point dependency to official release
• update glide.yaml accordingly

[codecov]

Codecov Report

Merging #434 into master will decrease coverage by 7.41%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #434      +/-   ##
==========================================
- Coverage   92.06%   84.64%   -7.42%     
==========================================
  Files          22       33      +11     
  Lines        1625     1498     -127     
==========================================
- Hits         1496     1268     -228     
- Misses         87      168      +81     
- Partials       42       62      +20

Impacted Files	Coverage Δ
zipkin.go
tracer.go
span.go
header.go
jaeger_thrift_span.go
tracer_options.go
interop.go
sampler.go
span_allocator.go
jaeger_tag.go
... and 45 more

---

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[yurishkuro]

codecov seems to be going insane

[yurishkuro]

Coverage change could be due to the fix to PACKAGE make var, which was previously empty, and potentially resulting in some coverage not being reported. Will need to sort it out separately, once there is a baseline.