MTELG: Add EventLoopGroup identifier (#1109)
MTELG: Add EventLoopGroup identifier (#1109)

Motivation:

With multiple MultiThreadedEventLoopGroups, the thread names are often
useless because there can be two NIO-ELT-#0s etc.

Modifications:

Add a unique MultiThreadedEventLoopGroup identifier.

Result:

NIO thread names unique. Side benefit: It should be easier to see if
people cycle through MTELGs like crazy.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio