[DBAL-237] Fix bug when removing and adding an index on the same columns that may be part of a foreign key.

[DBAL-204] Sane refactoring of the previous idea. Removed a bunch of code again and implemented proper schema namespacing. Also got the wording right (hopefully) and fixed the algorithms.

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

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

* The old implementation produced invalid SQLite statements for tables with
  a composite primary key, as SQLite cannot have more than one primary key
  clause and autoincrement can only be declared on a single column
* There is currently no way to tell from its pragma if an SQLite column
  auto-increments, therefore enabling it on all primary keys is not accurate
* SQLite primary keys auto-increment anyway (but slightly differently from an
  actual auto-increment column -- not guaranteed unique)