Add a grace time to the keystore TTL

Just to cover edge cases when sessions are about to expire.

server/http.go

@@ -203,7 +203,9 @@ func (h *Server) maybeUnlockKeystore(ctx context.Context, username, password str
 		}
 		shard = userinfo.Shard
 	}
-	return true, h.keystore.Open(ctx, shard, username, password, h.authSessionLifetime)
+	// Add a 'grace time' of 30 minutes to the key ttl.
+	ttl := h.authSessionLifetime + 1800
+	return true, h.keystore.Open(ctx, shard, username, password, ttl)
 }
 
 // Callback called by the login handler whenever a user successfully