Implement read-only realms

No docs changes because the implementation matches what the docs previously claimed.

Some assorted design decisions made in the process:

1. refresh and addNotificationBlock are considered "mutating" methods and disallowed on read-only realms on the grounds that they make no sense.

2. RLMRealm is split into RLMMutableRealm and RLMReadOnlyRealm. This is purely an implementation detail to slightly simplify the implementation and slightly improve performance (no checks for the readOnly property). It could be beneficial to expose this separation in the interface as they do have different APIs, but I didn't really feel it was worth it.

3. Missing tables in read-only realms are simply treated as if they were empty.  I wanted to make extra columns in tables or missing columns with default values treated similarly (so that you'd be able to open any realm file that either matches the schema or can be auto-migrated to the schema), but that'd require creating separate accessor classes and is far more complexity than it's worth.

4. No attempt is made at handling the case of a read-only RLMRealm in one process and a mutable RLMRealm in another process sharing a realm file. There's no easy solution, and more complex solutions (like putting a lock file in /tmp) should be done in the core and are not very high priority.

5. Realm files must already exist to be openable in read-only mode. Treating a nonexistent realm as an empty realm is theoretically doable, but would be awkward (and kinda dumb).
This is still called during migrations, so we should keep allowMutation as it was.