ENG-6884: Make all the compiler state consistent after parsing each DDL
statement.  The original thought was that it would be straightforward to
get just the changes for a statement from HSQL and apply these
statement-by-statement to state in the DDL compiler, but it turned out
uglier than originally thought.

ENG-6518: The ticket that prompted the above work.  Make EXPORT work
through live DDL and add tests.
Merge pull request #1787 from VoltDB/eng6884

ENG-6884 and ENG-6518.  r=jhugg.  May my ancestors forgive me...