Add a file for each PDO class (#3297)

Before, we had 5 classes in the ModelPdo file. It was bad for 2 reasons.
The first reason is that it is considered bad practice to have multiple
class in one file. This is especially true when using autoloading. On top
of that it is less readable considering the size of the file. The second
reason is that so far we were lucky. Everytime we needed to access the
database, it was through the ModelPdo class which loads all the other
classes. If we want to access directly the connection, it wont be loaded.
On top of that, the system is configured to work on a single database,
but as we have every connection definition in a single file, all classes
were loaded at the same time. Thus using memory and processing time for
nothing.
Now, we have a file for each class. To work with autoloading, classes
were slightly renamed to match autoloading rules.

docs/en/developers/Minz/migrations.md

@@ -18,7 +18,7 @@ Example:
 // File: app/migrations/2020_01_11_CreateFooTable.php
 class FreshRSS_Migration_2020_01_11_CreateFooTable {
 	public static function migrate() {
-		$pdo = new MinzPDOSQLite('sqlite:/some/path/db.sqlite');
+		$pdo = new Minz_PdoSqlite('sqlite:/some/path/db.sqlite');
 		$result = $pdo->exec('CREATE TABLE foos (bar TEXT)');
 		if ($result === false) {
 			$error = $pdo->errorInfo();