Fix Apple realtime with multiple subscriptions

[abnegate]

What does this PR do?

• BREAKING CHANGE: Realtime.subscribe is now async
• Fixes 🐛 Bug Report: 2nd Realtime subscription does not work sdk-for-apple#55
• Fixes 🐛 Bug Report: Xcode Data race with realtime - swift - sdk sdk-for-apple#56

Test Plan

TBD

Related PRs and Issues

• 🐛 Bug Report: 2nd Realtime subscription does not work sdk-for-apple#55
• 🐛 Bug Report: Xcode Data race with realtime - swift - sdk sdk-for-apple#56

Have you read the Contributing Guidelines on issues?

Yes

[stnguyen90]

A contributor tested something like this and the error that this was supposed to prevent came back:

NIOCore/EventLoop.swift:1168: Fatal error: BUG DETECTED: syncShutdownGracefully() must not be called when on an EventLoop.
Calling syncShutdownGracefully() on any EventLoop can lead to deadlocks.
Current eventLoop: SelectableEventLoop { selector = Selector { descriptor = 5 }, thread = NIOThread(name = NIO-ELT-0-#0) }

[lohanidamodar]

Left comment

[stnguyen90]

I was able to get multiple events through 🎉