Update default config for metadata cache.

* Enable connection metadata caching by default. Having it off can cause
  really poor performance on some MySQL configurations.
* Crank up the default cache duration to 2 minutes, so the metadata
  cache lasts longer.

Refs cakephp/cakephp#3446
Add longer caching duration when in debug=false mode.

This restores behavior that was present in 2.x where the cache duration
would extend for a very long time when debug=0.

Refs cakephp/cakephp#3446
Use ! instead of == false.
Fixed doc block
Merge pull request #54 from cakephp/orm-caching

Improve ORM caching