[PLAT-6799] Improve metadata performance with async file I/O

[nickdowell]

Goal

To reduce the overhead of adding metadata by performing filesystem I/O on a background queue.

Changeset

Metadata is now serialized to a JSON encoded C string in memory that is read at crash time.

File writing now occurs asynchronously on a dedicated serial dispatch_queue to free up the calling thread and avoid provoking filesystem contention.

Storage for the isLaunching property now goes through the KV-store (on the main thread) so that it can be accurately read at next start-up in the event of a crash (to compute lastRunInfo)

Testing

A unit test case has been added to verify that interrupted metadata updates do not result in invalid JSON being added to crash reports.

Ran successful full E2E on CI

[github-actions]

Infer: No issues found 🎉

OCLint: No issues found 🎉

Bugsnag.framework binary size increased by 1,744 bytes from 1,130,600 to 1,132,344

Generated by 🚫 Danger