Move SessionTracker IO operations to a BG thread

[philipphofmann]

Description

The SentrySessionTracker does a couple of file IO operations on the main thread. Even though they are lightweight, we should consider moving these to a BG thread. We have to ensure that the session tracking logic doesn't end up with race conditions and we have to ensure the logic doesn't break if the app crashes.

It's worth noting that a big customer reported this issue, and we have a customer case for this.

[linear[bot]]

COCOA-513 Move SessionTracker IO operations to a BG thread

[philipphofmann]

We see a couple of fatal app hangs in our internal SDK crash detection with the stack trace pointing to this root cause

Thread 0 Crashed:
0   libsystem_kernel.dylib          0x1e2ad20c8         write
1   Foundation                      0x1909b84c8         write
2   Foundation                      0x1909ba6ac         closure in writeToFileAux
3   Foundation                      0x1909b5da4         NSData._writeData
4   Foundation                      0x1909b583c         NSData._writeData
5   Sentry.framework                0x104ba21d8         -[SentryFileManager writeData:toPath:] (Sentry.framework:621)
6   Sentry.framework                0x104ba2ccc         -[SentryFileManager storeSession:sessionFilePath:] (Sentry.framework:879)
7   Sentry.framework                0x104ba0be4         -[SentryFileManager storeCurrentSession:] (Sentry.framework:308)
8   Sentry.framework                0x104ba8590         -[SentryHub storeCurrentSession:] (Sentry.framework:161)
9   Sentry.framework                0x104ba82a8         -[SentryHub startSession] (Sentry.framework:124)
10  Sentry.framework                0x104bd374c         -[SentrySessionTracker startSession]