pre: Some fixes to DB generators and temp tables support

While using temp tables in backup stuff, I've found some annoyances:

• concurrent (different connection) temp tables is not working ok in PostgreSQL, MSSQL and Oracle.
• sql generators have one PHP static cache that make things behave strange if 2 connections are used in the same request.

TODO:

• Add ddl tests about concurrent temp tables behavior.
• Fix concurrent temp tables behavior in PG, MSSQL and Oracle.
• Kill the static cache in generators. It really doesn't save too much (was useful before 1.9, when we were using it to guess object names, but now we are going against he dictionary).

Ciao