reset Context with subscription

[lolopinto]

we kinda support subscriptions by allowing subscription type to be set in the schema via https://github.com/lolopinto/ent/pull/1553

related task https://github.com/lolopinto/ent/issues/1544

ideally, we should automatically clear the context cache before each subscribe/resolve method since we could end up with long-lived subscriptions which can keep increasing the data in the context cache which is against the spirit of what it's designed for: was designed to live per-http-request

[lolopinto]

after https://github.com/lolopinto/ent/pull/1658, we don't want to clear context cache but reset the context so that each time, it's a new one