[ADQL] Fix table copy bug: the catalog and schema names where not copied or not interpreted when they were prefixing the new table name. WARNING: The copy function of a DBTable has a slightly different behaviour than before.

[TAP] Remake TAPFactory into an abstract class + let limit the number of jobs (async and sync) by allowing the TAPFactory to communicate with a connection pool

ADQL: Provide the position of any column reference (column in ORDER and GROUP BY) when returning a ParseException

ADQL: Fix an ADQL bug (raised by Dave Morris) in the management of subqueries: before, it was impossible to use (in a clause different from the FROM) columns of a father query inside a subquery.

ADQL: Completely change/improve Joins management, and particularly NATURAL JOIN and USING JOIN. Now, every joined columns are represented by a DBCommonColumn instance which has a given table coverage.
Before, there was a problem while using at least 3 or 4 NATURAL JOINs. Bug raised by Menelaos Perdikeas (ESAC).