Issue 565: MVStore: concurrently adding LOB objects (with MULTI_THREADED option) resulted in a NullPointerException. 

There was a way to prevent a database from being re-opened, by creating a column constraint that references a table with a higher id.

If a database file in the PageStore file format exists, this file and this mode is now used, even if the database URL does not contain "MV_STORE=FALSE". If a MVStore file exists, it is used.

Databases created with version 1.3.175 and earlier that contained foreign keys in combination with multi-column indexes could not be opened in some cases.

When using the PageStore, opening a database failed in some cases with a NullPointerException if temporary tables were used (explicitly, or implicitly when using large result sets).

MVStore: the ObjectDataType comparison method was incorrect if one key was Serializable and the other was of a common class.

MVStore: temporary tables from result sets could survive re-opening a database, which could result in a ClassCastException.

MVStore: temporary tables from result sets could survive re-opening a database, which could result in a ClassCastException.

MVStore: temporary tables from result sets could survive re-opening a database, which could result in a ClassCastException.

Undo change to make the tests run again. Sooner or later the MultiDimension tool will be removed as now we have spatial indexes.

fix NPE when backing up and there is a ".mv.db" file lying around that we do not currently have open