(dev/core#174) CRM_Utils_Cache_Interface::flush() - Return bool like PSR-16

(dev/core#174) CRM_Utils_Cache_Interface::flush() - Return bool like PSR-16

The function signature for `flush()` did not specify a return value, and
actual behaviors varied depending on the driver (`bool` for memcache,
`int` for redis, and `void` for others).

This standardizes on returning a bool -- which makes the signature align
with PSR-16's `clear()`.