Update _X_AMZN_TRACE_ID on every invoke

[bmoffatt]

Issue #, if available:

#236

Description of changes:

Updates the value of the environment variable _X_AMZN_TRACE_ID on every invoke. This is consistent with the docs

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[codecov-io]

Codecov Report

Merging #269 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #269      +/-   ##
==========================================
+ Coverage   74.59%   74.63%   +0.03%     
==========================================
  Files          20       20              
  Lines         681      682       +1     
==========================================
+ Hits          508      509       +1     
  Misses        128      128              
  Partials       45       45

Impacted Files	Coverage Δ
lambda/function.go	60.78% <100%> (+0.78%)	⬆️

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[alex]

I don't think this will work. The Go Lambda support uses net/rpc to server traffic. net/rpc spawns a go-routine per incoming connection.

Unless there's no concurrency at all here, because only a single connection is ever established, these environment variables will become a race condition, constantly stomping over each other between concurrent connections. Environemnt variables in general cannot be used for state for this reason.

If it really is the case that there's no concurrency, I'd implore you to add a comment next to the Setenv call.

[mhart]

A single Lambda (unix) process only ever processes one request at a time