Implement the IsDeferrable and InitiallyDeferred settings in the
Information_Schema.table_constraints table, plus test to show it works
as designed.
Cleanup and consolidate handling of ExistenceCheck
Support ALTER IF EXISTS
merge nathanlws: Support IF EXISTS clause on ALTER TABLE

Merge pull request #1356 from nathanlws/sql-layer/support-alter-if-exists
https://github.com/FoundationDB/sql-layer/pull/1356
merge tjoneslo: Implement the IsDeferrable and InitiallyDeferred settings in the

Merge pull request #1355 from tjoneslo/sql-layer/implement-is-deferrable-for-table-constraints
https://github.com/FoundationDB/sql-layer/pull/1355
fixed error message and associated tests
added new exception/error code, fixed tests
slight change to msg
forgot to add the exception class
merge hchandl: Improve error wording for not having aggregate function when using group by

Merge pull request #1358 from hchandl/sql-layer/errorMessageFix
https://github.com/FoundationDB/sql-layer/pull/1358
Merge branch 'master' into in-join-in