feat: catalogue generate now rate limits

Closes coop-cloud/organising#231.

pkg/limit/limit.go

@@ -0,0 +1,20 @@
+package limit // https://github.com/tidwall/limiter
+
+// Limiter is for limiting the number of concurrent operations. This
+type Limiter struct{ sem chan struct{} }
+
+// New returns a new Limiter. The limit param is the maximum number of
+// concurrent operations.
+func New(limit int) *Limiter {
+	return &Limiter{make(chan struct{}, limit)}
+}
+
+// Begin an operation.
+func (l *Limiter) Begin() {
+	l.sem <- struct{}{}
+}
+
+// End the operation.
+func (l *Limiter) End() {
+	<-l.sem
+}