fix: get stacktrace before any async method call

[olavloite]

The current stacktrace that is included in a SessionLeakError only contains the _acquire method itself, as the _acquire method calls an async method before getting the stacktrace. This change moves the call to get the caller stacktrace to the beginning of the method, which ensures that the stacktrace does contain the information that is needed.

This will also give us additional information that would be valuable to debug #716

Fixes #755

[codecov]

Codecov Report

Merging #756 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #756      +/-   ##
==========================================
+ Coverage   96.11%   96.11%   +<.01%     
==========================================
  Files          18       18              
  Lines       15620    15622       +2     
  Branches      532      532              
==========================================
+ Hits        15013    15015       +2     
  Misses        602      602              
  Partials        5        5

Impacted Files	Coverage Δ
src/session-pool.ts	99.2% <100%> (ø)	⬆️

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