If a server is removed during `HttpServerInventoryView.serverInventoryInitialized`,
the initialization gets stuck as this server is never synced. The method eventually times
out (default 250s).
Fix: Mark a server as stopped if it is removed. `serverInventoryInitialized` only waits for
non-stopped servers to sync.
Other changes:
- Add new metrics for better debugging of slow broker/coordinator startup
- `segment/serverview/sync/healthy`: whether the server view is syncing properly with a server
- `segment/serverview/sync/unstableTime`: time for which sync with a server has been unstable
- Clean up logging in `HttpServerInventoryView` and `ChangeRequestHttpSyncer`
- Minor refactor for readability
- Add utility class `Stopwatch`
- Add tests and stubs