Remove Constants class, remove LINQ attributes from models, get things compiling

- This is broken right now (it compiles, but won't run), I just wanted to start getting things into a base state we can figure a schema out from
- Constants is for something that's a RavenDB convention (i.e. keys are CollectionName/{0}/...), so remove it
- Since we're going to be redoing the schema, strip all the LINQ attributes from the entities
- Removed the ClaimId property from CassandraUserClaim and the Id property from CassandraUserLogin because we don't use them/need them
- Have CassandraUser just use the UserName as the Id (temporarily)
Start working on a new schema that implements the bare minimum needed for Identity support

- This doesn't compile yet because of the legacy code
- Commented out the other interfaces that we'll be bringing back one-by-one, implement just the IUserStore<TUser, TKey> interface in this commit
- Create a CassandraUser<TKey> class that only has the bare minimum properties necessary
- Denormalize users on write so they can be looked up by both Id and UserName
- Add a reference CQL script for the schema we're working with