Use dispatch_semaphore instead of NSCondition

[nickdowell]

Goal

Fixes the following issue observed in device logs for older iOS versions:

*** -[NSCondition dealloc]: condition (<NSCondition: 0x17411c200> '(null)') deallocated while still in use
*** Break on _NSLockError() to debug.

Also fixes an issue which can arise if the callback is called before waitUntilDate: - this does not work as expected with an NSCondition and results in a 2 second timeout.

GCD semaphores fix both of these issues.

Testing

Verified manually using example app.

[github-actions]

Infer: No issues found 🎉

OCLint: No issues found 🎉

Bugsnag.framework binary size increased by 232 bytes from 1,084,304 to 1,084,536

Generated by 🚫 Danger