Replace math/rand usage with crypto/rand

[ChrisRx]

This replaces the usage of math/rand with crypto/rand to support downstream usages of the random package that have security implications, such as the csrf middleware.

I don't see it used a lot of places, the only place outside of the csrf middleware appears to be the request_id middleware. If it makes more sense to ensure performance for non-crypographic usage of this package (which request_id middleware appears to be), I can create a second type and constructor for SecureRandom that uses the crypto/rand source, leaving the existing behavior for the Random type. Just let me know what you prefer.

[codecov-commenter]

Codecov Report

Merging #39 into master will decrease coverage by 0.14%. The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
- Coverage   59.26%   59.11%   -0.15%     
==========================================
  Files           6        6              
  Lines         518      521       +3     
==========================================
+ Hits          307      308       +1     
- Misses        208      209       +1     
- Partials        3        4       +1     

Impacted Files	Coverage Δ
random/random.go	87.50% <75.00%> (-12.50%)	:arrow_down:

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[ChrisRx]

I tried to make codecov happy locally, however, I think the difference it calculates in coverage is unavoidable. I think it is mistaken about the error being handled for the call to ReadByte(), because I believe the only safe thing to do should it return any error is for the program to crash. Is there anything that I need to do to help get this merged in?