Use [NSOperation start] instead of main for async NSOperation

[maksymmalyhin]

Fixes firebase/firebase-ios-sdk#8780.

Based on the NSOperation docs start method must be overridden for async operations and used instead of main.

[google-oss-bot]

Coverage Report

Affected SDKs

• GoogleDataTransport-ios-GoogleDataTransport.framework

  SDK overall coverage changed from 45.73% (886452d) to 55.52% (1b2dede) by +9.80%.

  Click to show coverage changes in 19 files.

  Filename	Base (886452d)	Head (1b2dede)	Diff
  GDTCCTCompressionHelper.m	87.14%	0.00%	-87.14%
  GDTCCTNanopbHelpers.m	58.82%	0.00%	-58.82%
  GDTCCTUploadOperation.m	91.17%	0.00%	-91.17%
  GDTCCTUploader.m	90.29%	16.00%	-74.29%
  GDTCORAssert.m	0.00%	88.89%	+88.89%
  GDTCORClock.m	69.47%	85.26%	+15.79%
  GDTCORConsoleLogger.m	20.69%	75.86%	+55.17%
  GDTCORDirectorySizeTracker.m	63.64%	90.91%	+27.27%
  GDTCOREndpoints.m	0.00%	100.00%	+100.00%
  GDTCOREvent+GDTCCTSupport.m	37.56%	0.00%	-37.56%
  GDTCOREvent.m	64.37%	97.70%	+33.33%
  GDTCORFlatFileStorage.m	14.51%	91.26%	+76.75%
  GDTCORLifecycle.m	42.17%	60.24%	+18.07%
  GDTCORPlatform.m	52.32%	62.13%	+9.81%
  GDTCORRegistrar.m	68.79%	69.50%	+0.71%
  GDTCORTransformer.m	0.00%	90.99%	+90.99%
  GDTCORTransport.m	0.00%	86.05%	+86.05%
  GDTCORUploadBatch.m	100.00%	0.00%	-100.00%
  GDTCORUploadCoordinator.m	61.40%	79.53%	+18.13%

Test Logs

• Base (886452d8): https://github.com/google/GoogleDataTransport/actions/runs/1347385960
• Head (1b2dede5): https://github.com/google/GoogleDataTransport/actions/runs/1347548246

[paulb777]

Please bump the patch version in the podspec and update the Changelog to prepare for a patch release.