server: shutdown endpoint (#466)

fixes #390

api/server/shutdown.go

@@ -0,0 +1,15 @@
+package server
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+func (s *Server) handleShutdown(halt context.CancelFunc) func(*gin.Context) {
+	return func(c *gin.Context) {
+		halt()
+		c.JSON(http.StatusOK, "shutting down")
+	}
+}