Increase DB connection pool size to 10

This is to address some errors we saw yesterday such as:

`sqlalchemy.exc.TimeoutError: QueuePool limit of size 5 overflow 10
reached, connection timed out, timeout 30`

Related flask-sqlalchemy docs:
http://flask-sqlalchemy.pocoo.org/2.3/config/#configuration-keys

manifest-api-base.yml

@@ -9,6 +9,7 @@ env:
   CW_APP_NAME: api
   # required by cf run-task
   FLASK_APP: application.py
+  SQLALCHEMY_POOL_SIZE: 10
 
   # Credentials variables
   ADMIN_BASE_URL: null