Overlapping dml ddl race condition

Fixes a race condition in `PostgresServerConnection#processParse()` where if a second thread completed a DDL during the parse processing, the parsed statement may end up with two (incorrect matching) row type schemas from two different AIS generations. 

The fix is adding line 1132 into `PostgresServerConnection#finishGenerating()`. 

I removed another set of unused rowtype Schema references, trying to make it clear that's not to be used anymore. 

Updated the `ConcurrentTestBuilderImpl` to use `RandomRule` to better reproduce problems with the MT tests. 
Dead