NIOSingletons: Use NIO in easy mode (#2471)

[1proprogrammerchant]

Motivation:

SwiftNIO allows and encourages to precisely manage all operating system resources like file descriptors & threads. That is a very important property of the system but in many places -- especially since Swift Concurrency arrived -- many simpler SwiftNIO programs only require a single, globally shared EventLoopGroup. Often even with just one thread.

Long story short: Many, probably most users would happily trade precise control over the threads for not having to pass around EventLoopGroups. Today, many of those users resort to creating (and often leaking) threads because it's simpler. Adding a .globalSingle static var which lazily provides singleton EventLoopGroups and NIOThreadPools is IMHO a much better answer.

Finally, this aligns SwiftNIO a little more with Apple's SDKs which have a lot of global singletons that hold onto system resources (Dispatch's thread pool, URLSession.shared, ...). At least in Dispatch's case the Apple SDKs actually make it impossible to manage the resources, there can only ever be one global pool of threads. That's fine for app development but wouldn't be good enough for certain server use cases, therefore I propose to add NIOSingletons as an option to the user. Confident programmers (especially in libraries) are still free and encouraged to manage all the resources deterministically and explicitly.

Companion PRs:

• NIOSingletonsTransportServices: Use NIOTS in easy mode apple/swift-nio-transport-services#180
• use NIOSingletons EventLoops/NIOThreadPool instead of spawning new swift-server/async-http-client#697

Modifications:

• Add the NIOSingletons type
• Add MultiThreadedEventLoopGroup.singleton
• Add NIOThreadPool.singleton

Result:

• Easier use of NIO that requires fewer parameters for users who don't require full control.
• Helps with ecosystem issue: EventLoopGroupProvider and the 'similar create or share ELG' pattern apple/swift-nio#2142
• Fixes NIOSingletons: Make EventLoopGroup shutdown throw apple/swift-nio#2472
• Partially addresses MultiThreadedEventLoopGroup: allow users to specify the thread name pattern apple/swift-nio#2473

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Motivation:

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Modifications:

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Result:

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