Front-end support for LIMIT ROWS DELETE statements

The first commit you have already seen here on GitHub.  The second completes the ticket by doing error checking, planning the statement, and logging the explained plan:
* The HSQL parser stores the text of the DELETE statement in the VoltXML that it generates
* `DDLCompiler` generates VoltXML from the text of the DELETE statement to verify that the statement is a DELETE that targets the correct table
  * The statement is then saved in the catalog as a child of the Table 
* After `VoltCompiler` compiles stored procedures, it proceeds to compile any DELETE statements designed to help enforce the ROW LIMIT constraint
  * It's at this time that the explained plan is logged.
Yes, you're right DDLCompiler might be a better place for the map.  VoltCompiler seems to be getting a little unwieldy.

I'm still a little reticent to refactor StatementCompiler and QueryPlanner.  In particular StatementCompiler does a lot of miscellaneous management that I'm wary of inadvertently tweaking.  I should probably just make the world a better place and do it.

It's funny you ask about trad DB.  I've had to give it thought because my test was failing in Jenkins---when the LIMIT EXECUTE test is run there, it fails because there is a schema verification pass that makes sure the canonical DDL produced by the catalog produces a catalog identical to the original DDL.  I didn't add code to generate the DELETE statement in CatalogSchemaUtils, so this was failing.  I am working on this now.

You made `ALTER` statements a condition for doneness in the catalog support ticket (ENG-7022).  It's looking like it *might* work out of the box, we'll see.