[ticket/12012] Correctly drop default value constraints on MSSQL

We need to drop the default constraints of a column,
before being able to change their type or deleting them.

PHPBB3-12012
[ticket/12012] Fix query layout

PHPBB3-12012
[ticket/12012] Drop and recreate indexes when changing a column on MSSQL

PHPBB3-12012
[ticket/12012] Fix tools::mssql_get_existing_indexes() for SQL Server 2000

PHPBB3-12012
[ticket/12012] Return SQL statements for index drop/create

Otherwise we recreate the index before changing the column

PHPBB3-12012
[ticket/12012] Add a unit test for changing the column type

PHPBB3-12012
[ticket/12012] Add a unit test for removing a column with indexes

PHPBB3-12012
[ticket/12012] Drop and recreate indexes when removing columns

PHPBB3-12012
[ticket/12012] Handle begin and commit transactions in tests

PHPBB3-12012
[ticket/12012] Remove duplicated code (only the $sql are different)

PHPBB3-12012
[ticket/12012] Fix docs in connection manager

PHPBB3-12012
[ticket/12012] Move MS SQL server comparison into a method

PHPBB3-12012
[ticket/12012] Move property to the top

PHPBB3-12012
Merge pull request #2288 from nickvergessen/ticket/12012

[ticket/12012] Correctly drop default value constraints

* nickvergessen/ticket/12012:
  [ticket/12012] Move property to the top
  [ticket/12012] Move MS SQL server comparison into a method
  [ticket/12012] Fix docs in connection manager
  [ticket/12012] Remove duplicated code (only the $sql are different)
  [ticket/12012] Handle begin and commit transactions in tests
  [ticket/12012] Drop and recreate indexes when removing columns
  [ticket/12012] Add a unit test for removing a column with indexes
  [ticket/12012] Add a unit test for changing the column type
  [ticket/12012] Return SQL statements for index drop/create
  [ticket/12012] Fix tools::mssql_get_existing_indexes() for SQL Server 2000
  [ticket/12012] Drop and recreate indexes when changing a column on MSSQL
  [ticket/12012] Fix query layout
  [ticket/12012] Correctly drop default value constraints on MSSQL