Remove security vulnerability that allowed connected users to read backend memory (Tom)

The vulnerability involves suppressing the normal check that a SQL function returns the data type it's declared to, or changing the data type of a table column used in a SQL function (CVE-2007-0555). This error can easily be exploited to cause a backend crash, and in principle might be used to read database content that the user should not be able to access.

Fix rare bug wherein btree index page splits could fail due to choosing an infeasible split point (Heikki Linnakangas)

Fix for rare Assert() crash triggered by UNION (Tom)

Tighten security of multi-byte character processing for UTF8 sequences over three bytes long (Tom)