ANNIS locks too many tables if querying multiple corpora

[thomaskrause]

When querying more than one corpus ANNIS will use the parent "facts" table in it's FROM clauses. This will acquire a lock on all child tables, even if they are excluded by the conditions. If you have more corpora imported than the value of the max_locks_per_transaction PostgreSQL parameter there will be an error. It is also bad for the statistics since PostgreSQL will use the overall statistics of all fact tables combined.

We should fix it in a way that only the fact-child tables that are really needed are queried.