Lock based leadership #606
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An in-progress notifications update allows injecting a callback module into `Postgrex.Notifications`. The callback can handle listeners and notifications however the application requires, and more importantly, it allows running arbitrary queries. By switching to our own notification implementation we get control over relaying messgaes to subscribers (rather than proxying them all through the connection) and keep a persistent connection for leadership elections.
The last remaining circuit breaker was used in the Postgres notifier, which no longer manages its own connection. Instead, the persistent connection manages auto reconnection by itself and between the `Connection.connected?` and `Senator.leader?` callbacks, plugins or other connection sensitve code can tailor behavior.
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Introduce a centralized leadership mechanism based on a single shared connection for simple queries and notifications.
This is largely functional, but still missing some error handling from the
Postgrex.Notificationsoverhaul, as well as some robustness to prevent the supervisor from crashing.Depends on elixir-ecto/postgrex#586
/cc @josevalim