Stop spawning Tasks inside of Combine sink closure.

[pixlwave]

#3699 highlighted an issue with this pattern where there's a race condition when

1. The Task uses the value from the publisher
2. The async method called awaits something and then uses that value.
3. The publisher is called multiple times in close succession.

This is obvious when you think about it, but this was the first instance where we saw the race condition in action. At this time, searching for the following regex reveals 27 instances of Tasks inside of .sinks.

\.sink\s*\{[^\}]*?(?:\s*\{[^\}]*?\}[^\}]*?)*\s*Task\s*\{(?:[^{}]*|\{[^{}]*\})*\}

Note about the regex:

It needs some work, as it never finishes searching (either in Xcode or via SwiftLint). A faster one is as follows, but it doesn't find instances where there's a block before the task:

\.sink\s*\{[^\}]*?Task\s*\{(?:[^{}]*|\{[^{}]*\})*\}

Looking at the 27 instances only 3 of them (at the time of writing) really meet the requirements for being an issue:

element-x-ios/ElementX/Sources/Services/Timeline/TimelineController/MockRoomTimelineController.swift

Lines 155 to 163 in a70189d

	signalCancellable = client.signals.sink { [weak self] signal in
	Task {
	do {
	try await self?.handleSignal(signal)
	} catch {
	MXLog.error(error.localizedDescription)
	}
	}
	}

element-x-ios/ElementX/Sources/UITests/UITestsNotificationCenter.swift

Lines 27 to 35 in a70189d

	signalCancellable = client.signals.sink { [weak self] signal in
	Task {
	do {
	try await self?.handleSignal(signal)
	} catch {
	MXLog.error(error.localizedDescription)
	}
	}
	}

element-x-ios/ElementX/Sources/Screens/BlockedUsersScreen/BlockedUsersScreenViewModel.swift

Lines 35 to 38 in a70189d

	.sink { [weak self] blockedUsers in
	guard let blockedUsers else { return }
	Task { await self?.updateUsers(blockedUsers) }
	}

We could go about fixing this in 2 ways:

• Re-write all of the sinks to use the for await in which would be tedious.
• Add an asyncSink method on Publisher that wraps everything up neatly make sure we finish processing one value before the next and use this instead.

Either way we should also add a SwiftLint rule to prevent us doing this in the future.