compile-time safety for cassandra queries

[sullivan-]

Right now, a number of queries will throw exception on cassandra. For instance, a query with an OR clause. Here's my idea for making this typesafe:

• add a type parameter to LongevityContext and Repo for the back end. Those types already exist in longevity.config.BackEnd. We will probably need to pull this as a configuration var at the same time. If so, we probably want to move that class hierarchy too - perhaps into longevity.context? Or into a new longevity.backend package.
• Create a type class OrSupport[_ <: BackEnd]. Give it a private[longevity] constructor so people can't create their own.
• Create implicit OrSupport[MongoDB], etc., for the back ends that support or queries. Put them where they will be found by implicit search - probably in the OrSupport companion object would be best.
• Add an implicit OrSupport[BE] argument for the "or" query methods - both in the query ADT and the query DSL.
• Repeat for other query clauses that don't work on Cassandra.
• Don't forget to delete the cassandra specific query exception classes