Rename persistent_id to persistentId.

The new name is more consistent with other settings in CakePHP.
Deprecate MemcacheEngine and update defaults for Memcached

People should switch to Memcached instead. The underlying extension is
better maintained and provides improved features and performance.

Collapse the persistent and persistentId settings, while also making
non-persistent connections the default. Persistent connections should be
an opt-in feature as having them enabled by default could go very wrong
on shared hosting environments.