Fixed #30171 -- Fixed DatabaseError in servers tests.

Made DatabaseWrapper thread sharing logic reentrant. Used a reference
counting like scheme to allow nested uses.

The error appeared after 8c775391b7.

tests/servers/test_liveserverthread.py

@@ -18,11 +18,10 @@ class LiveServerThreadTest(TestCase):
         # Pass a connection to the thread to check they are being closed.
         connections_override = {DEFAULT_DB_ALIAS: conn}
 
-        saved_sharing = conn.allow_thread_sharing
+        conn.inc_thread_sharing()
         try:
-            conn.allow_thread_sharing = True
             self.assertTrue(conn.is_usable())
             self.run_live_server_thread(connections_override)
             self.assertFalse(conn.is_usable())
         finally:
-            conn.allow_thread_sharing = saved_sharing
+            conn.dec_thread_sharing()