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 https://github.com/mParticle/mparticle-apple-sdk/blob/d604e598b74503ff6f675c0989a014a0f266f21d/mParticle-Apple-SDK/MPBackendController.m#L1986 b. on internal message queue, end old session and begin a new one in https://github.com/mParticle/mparticle-apple-sdk/blob/d604e598b74503ff6f675c0989a014a0f266f21d/mParticle-Apple-SDK/MPBackendController.m#L2200

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

[mparticle-automation]

:tada: This PR is included in version 8.21.0 :tada:

The release is available on GitHub release

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