Generalize platform SQL

[morozov]

Q	A
Type	improvement
BC Break	no

1. The CREATE DATABASE and DROP DATABASE statements are identical on all platforms that support databases.
2. The DBAL uses users on Oracle to emulate databases, so it should implement the "Create database" operation as CREATE USER. It already does the same with regards to dropping a database.
3. The IBM DB2 platform declares that it doesn't support databases:

   dbal/src/Platforms/DB2Platform.php

   Lines 440 to 443 in 525eda7

   	public function supportsCreateDropDatabase()
   	{
   	return false;
   	}

   but its test attempt to cover the generation of the CREATE DATABASE and DROP DATABASE statements:

   dbal/tests/Platforms/DB2PlatformTest.php

   Lines 350 to 351 in 11f0f94

   	self::assertEquals('CREATE DATABASE foobar', $this->platform->getCreateDatabaseSQL('foobar'));
   	self::assertEquals('DROP DATABASE foobar', $this->platform->getDropDatabaseSQL('foobar'));

4. The CREATE SCHEMA and DROP SCHEMA statements are identical on all platforms that support schemas.
5. The CREATE VIEW and DROP VIEW statements are identical on all platforms.
6. The is a common implementation of DROP SEQUENCE statements for all platforms that support sequences.

[morozov]

I don't think it's worth covering the paths where the NotSupported exceptions are thrown with tests. This is a side effect of the AbstractPlatform being a god object and declaring the super-set of the APIs that its child classes implement.

The proper way to address this issue is to take the gigantic API apart.