Pass agent context to subroutines

[sparrc]

Summary

Breaking out #2457 into smaller PRs.

This PR does not explicitly cancel the agent's context, but it passes it to more goroutines and fixes some of the waiting and concurrency logic. The routines that it passes to are:

1. spot instance draining poller
2. introspection http endpoint
3. task metadata http endpoint
4. engine event handler
5. tcs handler (already was passing but this change makes it wait and exit correctly when context is cancelled)

Description for the changelog

Fix agent top-level context

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.