Wrap write transactions in background tasks on supported platforms

[tgoyne]

This asks the OS to not suspend the app while a write is in progress, which can help prevent apps which share realm files between processes from getting "stuck" due to a suspended app holding the write lock. Because unconditionally wrapping things in tasks is extremely slow, it uses the app state change notification to only do so when the app is actually running in the background.

The manual testing I did for this (since it isn't really possible to write tests for) was:

1. Run performance tests
2. Run repro case from Deadlock race condition using realm within app group #4797 and verify that everything works as expected (on device only the active app makes forward progress, but I could never get the active app to get "stuck" by rapidly switching between the apps and/or killing them)
3. Run extension example on device and verify that still works.

Fixes #4797.

[bdash]

Won't this notification potentially be delivered on a different thread than the one this RLMRealm is associated with? I don't think it's safe to call -[RLMRealm inWriteTransaction] on a Realm from a different thread.

[tgoyne]

Right, I think it'll always be called on the main thread. inWriteTransaction doesn't check the thread so it'll usually work by coincidence, but it's a race condition.

[nyekimov]

we're using this approach and actually it's easy to receive Realm accessed from incorrect thread exception when trying to cancel a transaction in the write state.

[pauluhn]

2.6.0?

[jpsim]

No, this didn't make it into 2.6.0 because of the concern raised in https://github.com/realm/realm-cocoa/pull/4827/files#r110246828.

[egoldfarb]

I believe NSFileCoordinator is a better approach. See my comment:
#4797 (comment)

[tgoyne]

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