Refactor tests that use testutil.WaitForResult to use retry.
Since this requires refactoring the test functions in general this patch
also shows the use of the github.com/pascaldekloe/goe/verify library
which provides a good mechanism for comparing nested data structures.
Instead of just converting the tests from testutil.WaitForResult to
retry the tests that performing a nested comparison of data structures
are converted to the verify library at the same time.
This adds two goroutines to perform autopilot tasks on the leader - one
to monitor the health of servers and another to periodically clean up
dead servers with a limit on removal count. Also adds a new http endpoint,
`/v1/operator/autopilot/health`, for querying this information through an
operator RPC endpoint.
