mxresolv.Lookup() now properly randomized hosts in some situations

[thrawn01]

Purpose

mxresolv.Lookup() was not properly randomzing MX hosts returned in some situations.

Implementation

• Tests can now mxresolv.SetDeterministic() to improve the quality of testing
• Switched to mockdns for lookups instead of relying on actual DNS records on the internet.
• Now using r := rand.New(rand.NewSource()) instead of the global source in the rand package.
• Added tests to cover actual MX records found in the wild.
• Fixed an issue where the shuffle would ignore the last MX record in the last preference group.
• Fixed an issue where the shuffle would ignore the first MX record in the first preference group.
• Fixed an issue where prefer was ignored if there were only 2 MX records with difference preferences.
• Fixed an issue where the the first MX record would always be chosen if there were only 2 MX records with the same preference.
• Fixed https://github.com/mailgun/holster/issues/155
• Added benchmarks which show golang optimizing the rand.New() call.
• Disabled (gosec) linter for mxresolv as it complained I was using math/rand instead of crypto/rand

// pkg: github.com/mailgun/holster/v4/mxresolv
// BenchmarkShuffleWithNew
// BenchmarkShuffleWithNew-10       61962        18434 ns/op    5376 B/op       1 allocs/op
// BenchmarkShuffleGlobal
// BenchmarkShuffleGlobal-10        65205       18480 ns/op       0 B/op        0 allocs/op