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).

[laundmo]

I ended up writing a snippet like this to ensure it has an index:

dbstore = DBStore(db)

# ensure DBStore table has a index on rkey, as that can otherwise slow things down a lot

try:

dbstore.table.create_index(f"{dbstore.table._raw_rname}_rkey_idx", dbstore.table.rkey) # type: ignore

except RuntimeError:

pass # index already exists