[exporter/signalfx] Fix memory leaks

[crobert-1]

Description:

Changes in PR:

1. Add correlation client Shutdown function that blocks on the waitgroup. This is the main fix of this PR that should fix the leaking goroutines.
2. Re-organize the shutdown process of the apm client correlation test suite to properly synchronize the shutting down process.
3. Fix typo
4. Only block request sender until context is cancelled. The request processor is shutdown when the context is cancelled, so this would result in Shutdown waiting forever, since the request would never be processed.
5. Enable goleak in some more packages.

Note: This is contains the exact same contents as #30887, but change number 4 is new, and should resolve the test issue the original PR was causing.

Link to tracking Issue:
Resolves #30864
#30438

Testing:
All existing tests are passing, as well as added goleak checks. I'm going to run this a number of times to try to help ensure it's not flaky anymore.

[crobert-1]

This has passed 10+ times now without any failures. It should be ready to merge.