Move sqlutil.QuerySignature to apmsql

[axw]

Expose QuerySignature in the apmsql module.

Closes #513

[codecov-io]

Codecov Report

Merging #515 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #515      +/-   ##
==========================================
+ Coverage   84.09%   84.12%   +0.02%     
==========================================
  Files         114      114              
  Lines        6729     6729              
==========================================
+ Hits         5659     5661       +2     
+ Misses        766      765       -1     
+ Partials      304      303       -1

Impacted Files	Coverage Δ
module/apmsql/signature.go	100% <ø> (ø)
module/apmgorm/context.go	88.4% <100%> (ø)	⬆️
module/apmsql/driver.go	78.72% <100%> (ø)	⬆️
tracer.go	86.5% <0%> (+0.4%)	⬆️

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