This PR is a result/followup of the improvements we've made in the [SAMS repo](https://github.com/sourcegraph/sourcegraph-accounts/pull/199) that allows call sites to pass down a context (primarily to indicate deadline, and of course, cancellation if desired) and collects the error returned from `background.Routine`s `Stop` method.
Note that I did not adopt returning error from `Stop` method because I realize in monorepo, the more common (and arguably the desired) pattern is to hang on the call of `Start` method until `Stop` is called, so it is meaningless to collect errors from `Start` methods as return values anyway, and doing that would also complicate the design and semantics more than necessary.
All usages of the the `background.Routine` and `background.CombinedRoutines` are updated, I DID NOT try to interpret the code logic and make anything better other than fixing compile and test errors.
The only file that contains the core change is the [`lib/background/background.go`](https://github.com/sourcegraph/sourcegraph/pull/62136/files#diff-65c3228388620e91f8c22d91c18faac3f985fc67d64b08612df18fa7c04fafcd).
