Fix prepared statement caching bug.

It wasn't caching the prepared statement by keyspace, so if you performed
identical queries on two different keyspaces, it would use the prepared
statement from the wrong keyspace (and end up querying against the wrong
keyspace).

I added a "currentKeyspace" field to the Conn struct so the statement caching
code has it available to use as part of the cache key.
Merge pull request #222 from retailnext/fix-stmt-cache-key

Fix prepared statement caching bug.