Update PSR-16 library to use backend batch/clear implementations when available

[pgautier404]

The PSR-16 implementations of getMultiple, setMultiple, and deleteMultiple are currently client-side and the clear() method is unimplemented. There is a note in the library's README to set expectations about the resulting performance/availability of those methods. When the backend implementations are available, switch the client methods over and update the documentation.

[GrahamCampbell]

The multiple methods can be more efficiently implemented by using the async client methods from #154.