Using a Background Network Session for Crashlytics

In the network code, in `FIRCLSFABNetworkClient`, it appears to use `-[NSURLSessionConfiguration defaultSessionConfiguration]` as opposed to `-[NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:]`. In the past, I'd read in the old Crashlytics blog that Crashlytics used a background network session at the time. It was touted as a win for reliably sending up the crash reports. However, it appears that that has changed? If so, I'm curious why, and maybe it could be documented in `FIRCLSFABNetworkClient` to help others understand as well.