Add unique "delivery ID" to identify each attempt to process a message.

[jmalloc]

Partially addresses #68

[codecov-io]

Codecov Report

Merging #145 into master will decrease coverage by 0.03%.
The diff coverage is 31.57%.

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
- Coverage   29.25%   29.21%   -0.04%     
==========================================
  Files          74       75       +1     
  Lines        2694     2711      +17     
==========================================
+ Hits          788      792       +4     
- Misses       1882     1895      +13     
  Partials       24       24

Impacted Files	Coverage Δ
src/axrmq/marshaling.go	0% <0%> (ø)	⬆️
src/ax/endpoint/envelope.go	0% <0%> (ø)
src/ax/observability/logging.go	95% <100%> (+0.55%)	⬆️

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