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.
I think you want a "Statement?" vs. a "Statement*" to get the simplified accessor for "0 or 1" Statement vs. "0 or any number of Statements". Then you don't have to iterate through a trivial catalog map.

OTOH, I could envision a series of delete statements in a prioritized chain, so that if the first one has no effect, we'd try the next one, but that may be too overkill or too far in the future, so better to keep it simpler for now?