New SQL directive OPTION NOJOINPUSHDOWN that tells query planner to not push down any joins

This capability has been requested by a customer(see I-T ticket 217562). They want us to add in the following functionality:

-add an OPTION directive (OPTION NOJOINPUSHDOWN) to MetaMatrix SQL that tells the query planner whether or not to attempt to push down joins for a particular query. This would be a general purpose MetaMatrix SQL change. The default would be for our query planner to act as it does today (do join pushdown as allowed by connector capabilities), and the behavior if this OPTION directive was used would be to instead not do any join pushdown for any part of the user query, even if allowed by connector capabilities.

What this would allow them to do, if JBEDSP-348 is also implemented, is to leverage the limited join capability that SFDC has via its "relationship query" functionality. Because SFDC doesn't have full join capabilities, this would allow them to first try to submit a query against a VDB that uses SFDC as a source and see if the part of the join that the query planner pushes down to SFDC is something that SFDC can actually do. If not, the SFDC connector would throw an error, and the customer would resubmit their query with this OPTION NOJOINPUSHDOWN option.