Avoid defer when generating random number

[objectiser]

Signed-off-by: Gary Brown [email protected]

Which problem is this PR solving?

Remove defer used in generating random number, as recommended by @mandarjog here: a9d03b8#r31655784

Short description of the changes

Instead of putting generator back in pool using defer, do it explicitly.

[objectiser]

cc @mandarjog

[codecov]

Codecov Report

Merging #358 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #358      +/-   ##
==========================================
+ Coverage   87.22%   87.22%   +<.01%     
==========================================
  Files          54       54              
  Lines        3005     3006       +1     
==========================================
+ Hits         2621     2622       +1     
  Misses        272      272              
  Partials      112      112

Impacted Files	Coverage Δ
tracer.go	90.13% <100%> (+0.04%)	⬆️

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[black-adder]

do benchmarks show an improvment without the defer?

[objectiser]

@black-adder No real difference, but was a small test. However there seems to be evidence that defer is slower.