Lower EventLoopGroup requirements for creating bootstraps

[glbrntt]

Motivation:

In NIO ClientBootstrap and ServerBootstrap are initialized with an
EventLoopGroup. Since EventLoop conforms to EventLoopGroup you can
initialize an bootstrap with an existing event loop. In NIO Transport
Services the bootstraps are initialized with a NIOTSEventLoopGroup and
as NIOTSEventLoop conforms to EventLoop and by extension
EventLoopGroup (but not NIOTSEventLoopGroup) it is not possible to
initialize a bootstrap with a pre-existing NIOTSEventLoop.

Modifications:

Change the bootstrap initializers to accept an EventLoopGroup.

Result:

NIO Transport Services bootstraps can be initialized with a
NIOTSEventLoop.

[glbrntt]

cc @weissi

[weissi]

looks SemVer minor to me because this compiles:

protocol EventLoopGroup {}

struct NIOTSEventLoop: EventLoopGroup {}

func foo(_ c: EventLoopGroup) {}

let f: (NIOTSEventLoop) -> Void = foo

but @glbrntt to verify using NIO's check_no_api_breakages.sh

[weissi]

looks good to me! I don't really like the PR because it makes it easier to pass a wrong event loop that won't work but what you intend to do is entirely reasonable so this must work. Also it brings it more in line with NIO core. Real fix is SemVer major and needs to be designed alongside apple/swift-nio#674

[glbrntt]

Not a fan either but I don't know of a better way to do this!

I also added comments to the initialisers taking EventLoopGroup to explain their existence.

[weissi]

Thanks @glbrntt , having some explanation is great!

[glbrntt]

Also I tested API breakage on a local branch which removed commit afbbead because of #50 -- I can check again once #51 is merged if you like?