Add support for Opencensus Span links to Thrift Span references

[mayurkale22]

Fixes #472

[mayurkale22]

/cc @jindal93

[codecov-io]

Codecov Report

Merging #473 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #473      +/-   ##
==========================================
+ Coverage   94.76%   94.81%   +0.05%     
==========================================
  Files         150      150              
  Lines        9856     9900      +44     
  Branches      736      744       +8     
==========================================
+ Hits         9340     9387      +47     
+ Misses        516      513       -3

Impacted Files	Coverage Δ
src/common-utils.ts	95.23% <0%> (-0.6%)	⬇️
test/test-jaeger.ts	99.27% <0%> (+0.05%)	⬆️
test/test-tracecontext-format.ts	99.06% <0%> (+0.15%)	⬆️
src/tracecontext-format.ts	96.42% <0%> (+0.35%)	⬆️
src/jaeger-driver/index.ts	98% <0%> (+1.12%)	⬆️
src/stackdriver-monitoring.ts	82.02% <0%> (+3.37%)	⬆️

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