migrate\Plugin\migrate\id_map\Sql assumes a PDO db driver

Problem/Motivation

This code in Sql::ensureTables()

        try {
          $this->getDatabase()
            ->schema()
            ->createTable($this->mapTableName, $schema);
          break;
        }
        catch (DatabaseException $e) {
          $pdo_exception = $e->getPrevious();
          $mysql_index_error = $pdo_exception instanceof \PDOException && $pdo_exception->getCode() === '42000' && $pdo_exception->errorInfo[1] === 1071;
          $chunk_size--;
          // Rethrow the exception if the source IDs can not be in smaller
          // groups.
          if (!$mysql_index_error || $chunk_size <= 0) {
            throw $e;
          }
        }

assumes the DB driver is one implementing a PDO connection.

This blocks #3259709: Create the database driver for MySQLi that would be a non-PDO implementation.

Proposed resolution

Make that code db-driver abstract.

Implement two specific exceptions inheriting from SchemaException to cover the case of column too large and key too large, and throw them from mysql driver's ExceptionHandler. Change migrate's Sql to catch the new exceptions accordingly.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet