TQ: Start sled agents from background task (#9734)

When sleds are added to a trust quorum reconfiguration, and the
configuration has committed, any new sleds must have their subnets
allocated and sled agents started. We do that here from within the
background task that finishes the commit.

Author: andrewjstone

nexus/db-queries/src/db/datastore/trust_quorum.rs

@@ -787,7 +787,7 @@ impl DataStore {
         rack_id: RackUuid,
         epoch: Epoch,
         acked_commits: BTreeSet<BaseboardId>,
-    ) -> Result<(), Error> {
+    ) -> Result<TrustQuorumConfigState, Error> {
         opctx.authorize(authz::Action::Modify, &authz::FLEET).await?;
         let conn = &*self.pool_connection_authorized(opctx).await?;
 
@@ -873,9 +873,11 @@ impl DataStore {
                         )
                         .await
                         .map_err(|txn_error| txn_error.into_diesel(&err))?;
+
+                        return Ok(TrustQuorumConfigState::Committed);
                     }
 
-                    Ok(())
+                    Ok(TrustQuorumConfigState::Committing)
                 }
             })
             .await