add experimental support for thenables in the scope manager

[rochdev]

What does this PR do?

Adds experimental support for thenables in the scope manager.

Motivation

Users are having issues with thenables that are not native promises such has Knex because of an issue in Node.

[codecov-io]

Codecov Report

Merging #649 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #649      +/-   ##
==========================================
+ Coverage   96.14%   96.16%   +0.01%     
==========================================
  Files          92       92              
  Lines        3167     3181      +14     
==========================================
+ Hits         3045     3059      +14     
  Misses        122      122

Impacted Files	Coverage Δ
packages/dd-trace/src/config.js	100% <ø> (ø)	⬆️
packages/dd-trace/src/scope/async_hooks.js	95.91% <100%> (+1.63%)	⬆️
packages/dd-trace/src/noop/tracer.js	82.35% <100%> (ø)	⬆️
packages/dd-trace/src/tracer.js	93.33% <100%> (ø)	⬆️

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