Set statement timeout on all DB connections

A recent issue with a long-running query (#2288) highlighted the
fact that even though the original HTTP connection might be closed
(for example after gorouter timeout of 15 minutes, which returns a
504 response to the client), the request worker will not be stopped.

This means that the worker is spending time and potentially DB
resources generating a response that will never be delivered.

Gunicorn's timeout setting only applies to sync workers and there
doesn't seem to be an option to interrupt individual requests in
gevent/eventlet deployments.

Since the most likely (and potentially most dangerous) scenario for
this is a long-running DB query, we can set a statement timeout on
our DB connections. This will raise a sqlalchemy.exc.OperationalError
(wrapping psycopg2.extensions.QueryCanceledError), interrupting the
request after the given timeout has been reached.

This is a Postgres client setting, so the database itself will abort
the transaction when it reaches the set timeout.

Since this will also apply to our celery tasks (including potentially
long-running nightly tasks) we set a timeout of 20 minutes to begin
with.

This can potentially be split in the future to set a different value
for each app, so that we could limit API requests even more.

app/config.py

@@ -122,6 +122,7 @@ class Config(object):
     SQLALCHEMY_POOL_SIZE = int(os.environ.get('SQLALCHEMY_POOL_SIZE', 5))
     SQLALCHEMY_POOL_TIMEOUT = 30
     SQLALCHEMY_POOL_RECYCLE = 300
+    SQLALCHEMY_STATEMENT_TIMEOUT = 1200
     PAGE_SIZE = 50
     API_PAGE_SIZE = 250
     TEST_MESSAGE_FILENAME = 'Test message'