Using a NullPool for external connections by default (#4251)

Currently, even though `get_sqla_engine` calls get memoized, engines are
still short lived since they are attached to an models.Database ORM
object. All engines created through this method have the scope of a web
request.

Knowing that the SQLAlchemy objects are short lived means that
a related connection pool would also be short lived and mostly useless.
I think it's pretty rare that connections get reused within the context
of a view or Celery worker task.

We've noticed on Redshift that Superset was leaving many connections
opened (hundreds). This is probably due to a combination of the current
process not garbage collecting connections properly, and perhaps the
absence of connection timeout on the redshift side of things. This
could also be related to the fact that we experience web requests timeouts
(enforced by gunicorn) and that process-killing may not allow SQLAlchemy
to clean up connections as they occur (which this PR may not help
fixing...)

For all these reasons, it seems like the right thing to do to use
NullPool for external connection (but not for our connection to the metadata
db!).

Opening the PR for conversation. Putting this query into our staging
today to run some tests.

superset/models/core.py

@@ -639,7 +639,7 @@ def get_effective_user(self, url, user_name=None):
 
     @utils.memoized(
         watch=('impersonate_user', 'sqlalchemy_uri_decrypted', 'extra'))
-    def get_sqla_engine(self, schema=None, nullpool=False, user_name=None):
+    def get_sqla_engine(self, schema=None, nullpool=True, user_name=None):
         extra = self.get_extra()
         url = make_url(self.sqlalchemy_uri_decrypted)
         url = self.db_engine_spec.adjust_database_uri(url, schema)