Support other field storage backends than the default database backend

In order to support bigger websites with tailored needs, we need to make sure that NodeStream core supports other field storage backends than the default one.

This concerns all features except the product features currently bundled with core, since they aren't going to be part of NodeStream core for much longer. Each product can then support this if the need arises.

This is a meta-issue for issues like this, please comment here if you have any updates on issues that needs to be fixed in order for this to happen.

Some stuff that we need:
#1371520: Add EFQ Views to NodeStream