logger: do not leave sqlite transaction open

This should fix the following failure:

Traceback (most recent call last):
  File "bin/neostorage", line 23, in <module>
    neo.scripts.neostorage.main()
  File "neoppod/neo/scripts/neostorage.py", line 80, in main
    app.run()
  File "neoppod/neo/storage/app.py", line 167, in run
    logging.exception('Pre-mortem data:')
  File "/usr/lib/python2.6/logging/__init__.py", line 1088, in exception
    self.error(*((msg,) + args), **{'exc_info': 1})
  File "/usr/lib/python2.6/logging/__init__.py", line 1082, in error
    self._log(ERROR, msg, args, **kwargs)
  File "/usr/lib/python2.6/logging/__init__.py", line 1173, in _log
    self.handle(record)
  File "/usr/lib/python2.6/logging/__init__.py", line 1183, in handle
    self.callHandlers(record)
  File "neoppod/neo/lib/logger.py", line 210, in callHandlers
    self._queue(record)
  File "neoppod/neo/lib/logger.py", line 200, in _queue
    self._release()
  File "neoppod/neo/lib/logger.py", line 79, in _release
    self._async.pop()(self)
  File "neoppod/neo/lib/logger.py", line 96, in flush
    self.db.execute("BEGIN")
sqlite3.OperationalError: cannot start a transaction within a transaction

neo/lib/logger.py

@@ -93,10 +93,13 @@ class NEOLogger(Logger):
     def flush(self):
         if self.db is None:
             return
-        self.db.execute("BEGIN")
-        for r in self._record_queue:
-            self._emit(r)
-        self.db.commit()
+        try:
+            self.db.execute("BEGIN")
+            for r in self._record_queue:
+                self._emit(r)
+        finally:
+            # Always commit, to not lose any record that we could emit.
+            self.db.commit()
         self._record_queue.clear()
         self._record_size = 0