This is the OSS portion of enterprise PR 1994
Rather than directly interrogating the agent-local state for HTTP
checks using the `HTTPCheckFetcher` interface, we now rely on the
config snapshot containing the checks.
This reduces the number of changes required to support server xDS
sessions.
It's not clear why the fetching approach was introduced in
931d167ebb2300839b218d08871f22323c60175d.
Eric Haberkorn