Get rid of Context.Background()'s

[peterjan]

This PR gets rid of (almost) all uses of context.Background().
This should ensure renterd shuts down swiftly, providing a context is passed/used of course.

There are still bg ctxs in the following places:

• main()
• migrations
• tracing
• miner
• wallet
• bus setup

All of those are harmless imo but we could go all out... I also ignored all test files.

[ChrisSchinnerl]

Lgtm but can you test this on Arequipa? Just start it, count to 10 and then stop it and see if it shuts down any faster. Might also need to add some error handling to not spam our logs on every shutdown.