Do debouncing for autocomplete in a sane way
- Fixes https://github.com/vector-im/riot-web/issues/4419
- Fixes https://github.com/matrix-org/matrix-react-sdk/pull/518#issuecomment-285901871
- Fixes https://github.com/matrix-org/matrix-react-sdk/pull/518#issuecomment-285910503
- Fixes bug where the setting being used was the `autocompleteDelay` "syncedSetting" when it should have been the "localSetting" (so the setting being used was always the default)
