Increase DB connection pool size for API instances

We've seen the SQLAlchemy "could not acquire connection" error in
production during heavy traffic. Since we have more gunicorn eventlet
workers than we have DB connections available some workers need to
wait for a DB connection to become available before they can proceed
with the request. There's a timeout set on how long a worker would
wait and if that timeout is exceeded the above exception is raised.

Currently, we're using at most 1000 out of 5000 max DB connections,
40% peak CPU usage on the DB instance and an average of 60% CPU on
API instances during heavy load. The number of DB connections is
proportionally similar in preview and staging.

This slightly increases the number of max DB connections per API
instance. This should improve our utilization of API instances by
increasing the number of workers that can communicate with the DB
concurrently while staying well within the max DB connections limit.

manifest-api-base.yml

@@ -12,7 +12,7 @@ env:
   CW_APP_NAME: api
   # required by cf run-task
   FLASK_APP: application.py
-  SQLALCHEMY_POOL_SIZE: 15
+  SQLALCHEMY_POOL_SIZE: 20
 
   # Credentials variables
   ADMIN_BASE_URL: null