schedule,shared: move concurrency 429 limits into scheduler code (#849)

- make concurrency limiting the scheduler.Scheduler's responsibility
- eliminate the separate concurrency limit middleware 
- move concurrencyLimit logic into scheduler.FIFO to maintain backwards compatibility
- add HTTPError from #834 

Updates #834

internal/server/server.go

@@ -177,7 +177,6 @@ func (s *Server) routes() {
 	modelChain := chain.New(
 		authMW,
 		CreateRequestContextMiddleware(s.cfg),
-		CreateConcurrencyMiddleware(s.cfg),
 		CreateFilterMiddleware(s.cfg),
 		CreateFormFilterMiddleware(s.cfg),
 		CreateInflightMiddleware(s.inflight),