Fix expired security token issue

[eduardomourar]

Issue #, if available: #31

Description of changes:

In order to have a more robust solution for the internal calls to AWS, the following has been improved:

• Fix expired security token issue by always use fresh credentials provided by CloudFormation. We need to take into account the fact that lambda execution context are being reused, so ensure new values for credentials and settings are refreshed even when the BaseResource instance is the same from previous call.
• Use queue to avoid throttling of AWS API calls (specially CloudWatch).
• Optional use of worker threads for performance reasons.

[codecov]

Codecov Report

Merging #30 (5bcf8c3) into master (4594481) will decrease coverage by 0.78%.
The diff coverage is 96.52%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
- Coverage   99.71%   98.93%   -0.79%     
==========================================
  Files          13       13              
  Lines        1057     1218     +161     
  Branches      185      212      +27     
==========================================
+ Hits         1054     1205     +151     
- Misses          3       13      +10     

Flag	Coverage Δ
unittests	98.93% <96.52%> (-0.79%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/utils.ts	93.93% <91.89%> (-6.07%)	⬇️
src/log-delivery.ts	98.91% <97.11%> (-0.70%)	⬇️
src/resource.ts	98.38% <97.56%> (-0.67%)	⬇️
src/interface.ts	100.00% <100.00%> (ø)
src/metrics.ts	100.00% <100.00%> (ø)
src/proxy.ts	100.00% <100.00%> (ø)

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