only require userland CLS when needed

[vmarchaud]

We shipped our APM with the @opencensus/core package. We didn't enable the tracer by default but since when required, it will require the continuation-local-storage which will apply patches automatically. So even if you use async_hooks, you will have the patchs from the userland CLS.
I believe it's safe to change this behavior since it will work the same way.

I would like to raise also the fact that we could only require both CLS implementation when the tracing is enabled to decrease the performance impact when not enabled.

If you are interested, here are the different complaints from users: keymetrics/pm2-io-apm#235

[mayurkale22]

Please rebase the branch to fix build.

[codecov-io]

Codecov Report

Merging #435 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #435      +/-   ##
==========================================
- Coverage   95.01%   94.98%   -0.04%     
==========================================
  Files         147      147              
  Lines        9545     9544       -1     
  Branches      679      679              
==========================================
- Hits         9069     9065       -4     
- Misses        476      479       +3

Impacted Files	Coverage Δ
src/stackdriver-monitoring.ts	80.2% <0%> (-3.13%)	⬇️
src/internal/cls.ts	81.81% <0%> (-1.52%)	⬇️

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[mayurkale22]

LGTM