Holding keys and their prefixes as "bytes" is error prone. It's easy
to mix them up (or use wrong types). This change adds wrappers for
keys with accessors which are meant to make misuses as difficult as
possible.
Prefix and full keys are now distinguished. Places which assumed that
the representation is the same (it currently is) were changed not to
do so. This will allow us to introduce more compact storage for non-prefix
keys.
validate_cql_key() is based on validate_key() from ThriftValidation. I
didn't name it thrift_validation.hh though because it's meant to work
on CQL model, not thrift. For instance, it's using our
schema::partition_key_type, which is not thrift-compatible.
