Replace use of db.conn.close() as it is not supported in the new backend

src/calibre/db/__init__.py

@@ -135,7 +135,7 @@ Various things that require other things before they can be migrated:
     1. From initialize_dynamic(): Also add custom
                     columns/categories/searches info into
                     self.field_metadata.
-    2. Port library/restore.py
+    2. Port library/restore.py, check_library.py and reinit_db() from debug.py
     3. Check that content server reloading on metadata,db change, metadata
     backup, refresh gui on calibredb add and moving libraries all work (check
     them on windows as well for file locking issues)

src/calibre/gui2/dialogs/check_library.py

@@ -45,7 +45,7 @@ class DBCheck(QDialog): # {{{
         self.user_version = self.db.user_version
         self.rejected = False
         self.db.clean()
-        self.db.conn.close()
+        self.db.close()
         self.closed_orig_conn = True
         t = DBThread(self.db.dbpath, False)
         t.connect()
@@ -80,7 +80,7 @@ class DBCheck(QDialog): # {{{
             self.pb.setMaximum(self.count)
             self.pb.setValue(0)
             self.msg.setText(_('Loading database from SQL'))
-            self.db.conn.close()
+            self.db.close()
             self.ndbpath = PersistentTemporaryFile('.db')
             self.ndbpath.close()
             self.ndbpath = self.ndbpath.name

src/calibre/gui2/dialogs/restore_library.py

@@ -103,7 +103,7 @@ def restore_database(db, parent=None):
                 'blank list of books.'
                 '<p>Do you want to restore the database?')):
         return False
-    db.conn.close()
+    db.close()
     d = DBRestore(parent, db.library_path)
     d.exec_()
     r = d.restorer

src/calibre/gui2/ui.py

@@ -622,7 +622,7 @@ class Main(MainWindow, MainWindowMixin, DeviceMixin, EmailMixin,  # {{{
         if olddb is not None:
             try:
                 if call_close:
-                    olddb.conn.close()
+                    olddb.close()
             except:
                 import traceback
                 traceback.print_exc()

src/calibre/library/restore.py

@@ -130,7 +130,7 @@ class Restore(Thread):
                                  restore_all_prefs=True,
                                  progress_callback=self.progress_callback)
             db.commit()
-            db.conn.close()
+            db.close()
             self.progress_callback(None, 1)
             if 'field_metadata' in prefs:
                 self.progress_callback(_('Finished restoring preferences and column metadata'), 1)
@@ -232,7 +232,7 @@ class Restore(Thread):
             for i,args in enumerate(self.custom_columns.values()):
                 db.create_custom_column(*args)
                 self.progress_callback(_('creating custom column ')+args[0], i+1)
-        db.conn.close()
+        db.close()
 
     def restore_books(self):
         self.progress_callback(None, len(self.books))
@@ -252,7 +252,7 @@ class Restore(Thread):
             db.conn.execute('UPDATE authors SET link=? WHERE name=?',
                             (link, author.replace(',', '|')))
         db.conn.commit()
-        db.conn.close()
+        db.close()
 
     def restore_book(self, book, db):
         db.create_book_entry(book['mi'], add_duplicates=True,