revamped rowcache. Drop support for anything that requires nested
lookups. Use consolidated IN clauses for tables that have simple
primary keys.
Merge branch 'resharding'
Removing unused SessionParams attributes.

The python code only uses Keyspace and Shard.
That also allows me to remove KeyRange from dbconfigs,
was only used to validate against the connection params.
(the other one, Dbname, is used in other places).