fix: Correct threading around user notification log

[BrandonStalnaker]

Instructions

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Summary

• Fix occasion timing crash around log notification

1. App goes to background for a long time (not killed)
2. User receives and taps notification
3. App opens and at the same, concurrently, time attempts to
   a. on main queue, log user notification with _session property in

   mparticle-apple-sdk/mParticle-Apple-SDK/MPBackendController.m

   Line 1986 in d604e59

   MPMessageBuilder *messageBuilder = [MPMessageBuilder newBuilderWithMessageType:MPMessageTypePushNotification session:_session messageInfo:messageInfo];

   
   b. on internal message queue, end old session and begin a new one in

   mparticle-apple-sdk/mParticle-Apple-SDK/MPBackendController.m

   Line 2200 in d604e59

   [self endSessionIfTimedOut];

The read on main queue in logUserNotification: method is done without synchronisation, resulting in occasional crashes on notification interactions.

Testing Plan

• Tested using sample app while manually triggering logNotification

Reference Issue (For mParticle employees only. Ignore if you are an outside contributor)

• Closes https://go.mparticle.com/work/SQDSDKS-6192

[einsteinx2]

Looks like a solid solution to the issue.

[mparticle-automation]

🎉 This PR is included in version 8.21.0 🎉

The release is available on GitHub release

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