New IMPLICITLY_COMMITTED error
New TransactionMode, use in DDL statements
Check for new mode, warn and commit
Test it
Minor existing test adjustment
Guard against improper checkTransactionMode usage
merge nathanlws: DDL causes implicit commit

Merge pull request #1126 from nathanlws/sql-layer/ddl-causes-implicit-commit
https://github.com/FoundationDB/sql-layer/pull/1126