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.