fatalError() during high concurrency test

[vlm]

Expected behavior

Users generally expect Swift NIO not to fail with internal assertions and fatalErrors they couldn't catch.

Actual behavior

Fatal error: Invalid FSM transition attempt: state complete, input connectTimeoutElapsed: file swift-nio/Sources/NIO/HappyEyeballs.swift, line 417
Illegal instruction: 4

Steps to reproduce

Compile and run the attached code against the master.

If possible, minimal yet complete reproducer code (or URL to code)

reproduce.swift.

SwiftNIO version/commit hash

b8d883da2c2090d02ec4890b5ab5c7c9bfa30935

Swift & OS version (output of swift --version && uname -a)

Apple Swift version 4.1 (swiftlang-902.0.38 clang-902.0.30)
Target: x86_64-apple-darwin17.4.0
Darwin Levs-MacBook-Pro.local 17.4.0 Darwin Kernel Version 17.4.0: Sun Dec 17 09:19:54 PST 2017; root:xnu-4570.41.2~1/RELEASE_X86_64 x86_64

[Lukasa]

Should be fixed by #191.