Add deprecation for old usages of cachable and readOnly in CPs

* Passing opts.cacheable to the constructor of ComputedProperty is deprecated
* Passing opts.readOnly to the constructor of ComputedProperty is deprecated
* Invoking `cacheable` in a computed property is preprecated.
* Invoking `readOnly` with aany params in a computed property is deprecated.

-- UPDATE --

After reading #9290 and https://github.com/emberjs/rfcs/pull/12, seems that this PR is the first step to make CP read only by default.

Steps done in this PR:

- [x] Raise deprecation warnings when passing `cacheable` or `readOnly` to the constructor. No extrictly necessary, but is not used anywhere in the code since 2012 and simplifies things.
- [x] Deprecate usage of `cacheable()` and `readOnly()`. Are the default.

Steps pending:
- [ ] Modify CPs to work internally with setter and getters. A way of doing that gracefully without people even noticing it could be count the arity of the function passed to `Ember.computed`. If is arity <=1, create only a setter. If has arity > 1, assign the same function to getter and setter. Passing an object also works. No deprecations yet.
- [ ] Deprecate passing functions with arity > 1. Recommend user `{get, set}` instead.
- [ ] Implement logic in this RFC: Create `writable` and/or `overridable` and determina default values as explained in the code example above.
- [ ] Deprecate all the things!
- [ ] Ember 2.0. Delete all the things!

This steps don't need to happen in this PR. I fact I would merge things one step at a time.


So what do you suggest then? No mention volatile at all?