Implemented support for persistent connections in PDO and mysqli drivers

Fix UniqueConstraint with empty name.

Added type hints to the Column class

Drop support for feferred misspelling in PostgreSqlPlatform::getAdvancedForeignKeyOptionsSQL()

PostgreSqlPlatform::getAdvancedForeignKeyOptionsSQL()

Reworked AbstractSchemaManager::extractDoctrineTypeFromComment(), removed ::removeDoctrineTypeFromComment()

Removed errorCode() and errorInfo() methods from Connection and Statement APIs

Reworked `AbstractSchemaManager::extractDoctrineTypeFromComment()` removed `::removeDoctrineTypeFromComment()`

Reworked driver exceptions

Removed DB2SchemaManager::_getPortableForeignKeyRuleDef()

Enable strict types

Reworked AbstractPlatform::get*Expression() methods

Modified AbstractPlatform::getLocateExpression() and ::getSubstringExpression() signatures

Changed the type of $char in AbstractPlatform::getTrimExpression()`from string|false to ?string

Connection::quote() can only quote strings

Changed the type of `$char` in `AbstractPlatform::getTrimExpression()` from `string|false` to `?string`

1. The argument is always available since the method is only called from listTableIndexes() which requires a table name.
2. The argument itself seems a workaround and only needed to bypass the fact that the SQLiteSchemaManager violates the method contract: instead of reformatting the provided data it fetches more data from the DB schema which requires a table name. This argument should be dropped completely later.

Converted Connection and Statement methods which returned false in case of a failure into void

Moved rowCount() from Statement to ResultStatement

Transaction-related Statement methods return void

Statement::fetchColumn() will throw an exception in the case of invalid index