Replace ConcurrentQueue<T> in BackgroundWorker

[jamescrosswell]

Resolves #2516

That issue has quite a few messages in the history but see here for what finally gave rise to this PR:

• SentryEvent memory leak with large exception object graphs #2516 (comment)

Memory profile

Before this PR

After this PR

Performance

I was a worried about performance using a bespoke concurrent queue but we seem to have pretty comparable performance to ConcurrentQueue<T> in our specific use case (a bit better in fact).

Method	N	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated
ConcurrentQueueLiteAsync	1000	4.106 ms	0.0962 ms	0.2835 ms	1074.2188	507.8125	23.4375	2.18 MB
ConcurrentQueueAsync	1000	5.073 ms	0.1282 ms	0.3781 ms	1062.5000	515.6250	125.0000	2.21 MB

I tried a few different variations of the code (using a LinkedList<T> and Queue<T> for the internal queue) bit plain old List<T> turned out to be the fastest by quite a margin (and use the least memory).

[vaind]

Looks pretty good to me except for the tests that are a bit sparse. I've added som suggestions although they're not exhaustive.

[vaind]

LGTM except for the unrelated sentry-cocoa change.

Also, should we change breadcrumbs too in a followup PR? That's the only other place we use concurent queue in (other than tests)

[bruno-garcia]

Can we release this asap?

[bruno-garcia]

It's out on: https://github.com/getsentry/sentry-dotnet/releases/tag/4.6.0