WIP - 3.0 - Connection prefix

As mentionned in #3843, a fresh start to implement the connection prefix feature.
At this point, this commit puts the PR back to the state it was, but with an up to date Cake 3.0 version (and better managed branches on my side)

Refs #2666 and #4118 

To do :
- Update Schema system so it takes into account the connection prefix (if any)
- Maybe update the \ORM\Table class (I have a written note about that on my desk, so I guess "past me" put the finger on something I may have forgotten)
- Make Fixtures apply connection prefixes

I'll probably have some questions about testing with / without a prefix and making the fixtures apply the prefix. I know I got stuck on something regarding this but can't exactly remember what. I'll dive in this again and ask you as they come up.
If that's the case, it's settled. I looked in the docs, but couldn't find it ([Coding Standards](http://book.cakephp.org/3.0/en/contributing/cakephp-coding-conventions.html#commenting-code)), maybe that section needs updating.