LP:809849 - spatial operations must be KILL-able

Spatial operations can be very CPU intensive, and they are not killable using KILL if they do not use any table rows. This causes the following problems:

• The RQG can not terminate randomly-generated spatial expressions that take too long to calculate, causing the test to block until the expression is calculated;
• It is possible to DoS a server using a spatial expression. No table or update rights are required – just the ability to connect to the server are required;
• trying to work around the limitations of SQL and the OpenGIS model via application-side logic or CONCAT tricks may produce a query that does not use any tables and is thus unkillable.