close sqlalchemy session after a worker's work. closes #4

that was far easier than i anticipated...

tasks.py

@@ -1,4 +1,4 @@
-from celery import Celery
+from celery import Celery, Task
 
 from app import app as flaskapp
 from app import db
@@ -31,6 +31,16 @@ if 'SENTRY_DSN' in flaskapp.config:
     register_logger_signal(sentry)
     register_signal(sentry)
 
+
+class DBTask(Task):
+    def __call__(self, *args, **kwargs):
+        try:
+            super().__call__(*args, **kwargs)
+        finally:
+            db.session.close()
+
+app.Task = DBTask
+
 @app.task(autoretry_for=(TwitterError, URLError))
 def fetch_acc(id, cursor=None):
     acc = Account.query.get(id)