(dev/core#174) CRM_Utils_Cache_Interface::get() should match PSR-16

(dev/core#174) CRM_Utils_Cache_Interface::get() should match PSR-16

Comparing `CRM_Utils_Cache_Interface::get()` and `Psr\SimpleCache\CacheInterface::get()`,
they agree on key details:

1. They return values are `mixed` (strings, arrays, numbers,, etc)
2. The default representation of a cache-miss is NULL.

They differ in that PSR-16 allows the caller to optionally specify a `$default`.

Since no existing callers actually use this, we can (for moment) throw an
error if someone tries to pass `$default` to a driver that doesn't support
it.

See also: https://www.php-fig.org/psr/psr-16/