[DBAL-195] Fix AbstractSchemaManagertests/Doctrine/Tests/DBAL/Functional/Schema/SchemaManagerFunctionalTestCase.phpdropAndCreateSequence

Schema events

Reopened on feature branch: Fix Postgres Date Add/Sub functions for dynamic values

fixed documentation mismatch

Add getGuidExpression() to AbstractPlatform, already exists in MySQLPlatform

These are internal, automatically-created and shouldn't be considered part of
the schema. They cannot be recreated because of their reserved names.