Adding pessimistic sqlalchemy disconnection handling

lemur/extensions.py

@@ -3,7 +3,15 @@
     :copyright: (c) 2018 by Netflix Inc., see AUTHORS for more
     :license: Apache, see LICENSE for more details.
 """
-from flask_sqlalchemy import SQLAlchemy
+from flask_sqlalchemy import SQLAlchemy as SA
+
+
+class SQLAlchemy(SA):
+    def apply_pool_defaults(self, app, options):
+        SA.apply_pool_defaults(self, app, options)
+        options["pool_pre_ping"] = True
+
+
 db = SQLAlchemy()
 
 from flask_migrate import Migrate