Introduce transaction support

[k00ni]

Transaction support would allows you to group SPARQL commands and rollback, if an error occurs while the transaction is still open. Very helpful in scenarios, in which you rely on a consistent database.

Unfortunately, this doesn't work with MyISAM tables, which means, as long as ARC2 uses MyISAM tables per default, transaction support via PDO is not possible. Source

• requires introduction of PDO - see #109
• requires change of table engines from MyISAM to InnoDB (breaks backward compatibility, requires fresh install or manually migration) - #115
• functions
  • beginTransaction
  • inTransaction
  • commit
  • rollback
• check handling of sub-transaction

[k00ni]

Done.