sql.py: DbConn: Added is_cached(). run_query(): Use new DbConn.is_cached() to avoid creating a savepoint if the query is cached.

sql.py: DbConn: Also cache cursor.description

sql.py: DbConn: Cache query results as a dict subset of the cursor's key attributes, so that additional attributes can easily be cached by adding them to the subset list

dicts.py: Added AttrsDictView

util.py: NamedTuple.__iter__(): Removed unnecessary **attrs param

sql.py: _query_lookup(): Fixed bug where params was cast to a tuple, even though it could also be a dict. index_cols(): Changed to use the connection-wide caching mechanism rather than its own custom cache.

util.py: NamedTuple: Made it usable as a hashable dict (with string keys) by adding iter() and getitem()

dicts.py: Added make_hashable()

sql.py: DbConn: Only cache exceptions for inserts since they are not idempotent, but an invalid insert will always be invalid. If a cached result in an exception, re-raise it in a separate method other than the constructor to ensure that the cursor object is still created, and that its query instance var is set.

sql.py: insert(): Cache insert queries by default. This works because any DuplicateKeyException, etc. would be cached as well. This saves many inserts for rows that we already know are in the database.