provide graceful shutdown interface

This introduces a mechanism for politely asking Underway to shutdown. To
do so, a new function, `graceful_shutdown` is provided. Calling this
function will send a notification to a Postgres channel. Workers listen
on this channel and when a message is received will stop processing new
tasks. If they're already processing a task, then they wait until that
task is done or the task timeout has elapsed, whichever is first.

In order to cleanly stop the queue, this function should be used. If
stopping in-progress tasks is safe for your use case, then this can be
ignored and the queue can be stopped without any delay.

Closes #5

src/queue.rs

@@ -959,6 +959,28 @@ impl<T: Task> QueueBuilder<T, PoolSet> {
     }
 }
 
+pub(crate) const SHUTDOWN_CHANNEL: &str = "underway_shutdown";
+
+/// Initiates a graceful shutdown by sending a `NOTIFY` to the
+/// `underway_shutdown` channel via the `pg_notify` function.
+///
+/// Workers listen on this channel and when a message is received will stop
+/// processing further tasks and wait for in-progress tasks to finish or
+/// timeout.
+///
+/// This can be useful when combined with [`tokio::signal`] to ensure queues are
+/// stopped cleanly when stopping your application.
+pub async fn graceful_shutdown<'a, E>(executor: E) -> Result
+where
+    E: PgExecutor<'a>,
+{
+    sqlx::query!("select pg_notify($1, $2)", SHUTDOWN_CHANNEL, "")
+        .execute(executor)
+        .await?;
+
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests {
     use std::collections::HashSet;