Do not store protocol in the database

[aristath]

We should not store the protocol of files in the database.
Instead of http://mysite.com/wp-content/uploads/image.png we could strip the protocol while saving the entry and save it as //mysite.com/wp-content/uploads/image.png instead.
This will make http/https issues a lot easier to address, and hopefully ease migrations to HTTPS.
There are a bunch of plugins out there that can do this, but they shouldn't have to... This is something that can be done in-core a lot more efficiently for everyone.