DBStore (session storage) should probably create index on rkey

[laundmo]

Depending on the expiration, which defaults to "never", this table can grow quite large if a site is used regularly. That makes DBStore.get very slow if the table is large, due to the DB needing to do a full scan when querying for table.rkey == key.
 
To avoid users needing to manually create a index on rkey, the DBStore should probably do this automatically (while not forgetting that the create_index method internally does a commit/rollback).