fix: Re-work Sentry minidump loader

[timfish]

Closes #763

This PR re-writes the minidump loading code.

• All the loading, validation, deleting and error catching logic is now all contained in createMinidumpLoader.
• Removes possible race conditions from overlapping calls to lookup minidumps via our existing Mutex
• Only reads minidumps from disk when they haven't been modified for 1 second
• Skips sending minidumps if they're too small or don't start with the expected minidump header
• Adds unit tests for the new logic

I had to extend the timings on many of the e2e tests because these changes add a delay in minidump sending while we wait to ensure the file is not being written.

[AbhiPrasad]

I'm confused at the extra session, but I think we should just ship since it doesn't break anything

[timfish]

My best guess is that the extra session has always been there but we're only seeing it now after extending the app run time to cater for the sending delays added by this PR