Add cloneTable() to DatabaseSchema

The DatabaseSchema added a new method and abstract method:

/**
 * Create an exact duplicate of an existing table, including all content.
 *
 * @param $source
 *   The name of the table to be copied.
 * @param $target
 *   The name of the new table to be created and populated with data.
 *
 * @throws DatabaseSchemaObjectExistsException
 *   If the new table specified already exists.
 */
public function cloneTable($source, $target) {
  if ($this->tableExists($target)) {
    throw new DatabaseSchemaObjectExistsException(t('Table @name already exists.', array('@name' => $target)));
  }
  $statements = $this->cloneTableSql($source, $target);
  foreach ($statements as $statement) {
    $this->connection->query($statement);
  }
}

/**
 * Generate an array of query strings suitable for cloning a table.
 *
 * @param $source
 *   The name of the table to be copied.
 * @param $target
 *   The name of the new table to be created and populated with data.
 * @return
 *   An array of SQL statements to clone the table, including inserting data.
 */
abstract protected function cloneTableSql($source, $target);