ZSQLCatalog: Use a transactional cache instead of a global cache.

Not perfect yet (what if definitions change during the same transaction ?),
but better than previous state.
ZSQLCatalog: Only strip value when used
ZSQLCatalog: Also cache misses.

There are a lot of misses in this function because related keys must have
precedence over homonym columns, which makes it cost a *lot* of cpu time
to just return None.