[#13] Rename go module name according to NSPCC standards

- refactoring s3 gate structure
- cleanup unused code
- rename go module to `github.com/nspcc-dev/neofs-s3-gate`

closes #13

Signed-off-by: Evgeniy Kulikov <kim@nspcc.ru>

api/max-clients.go

@@ -0,0 +1,56 @@
+package api
+
+import (
+	"net/http"
+	"time"
+)
+
+type (
+	MaxClients interface {
+		Handle(http.HandlerFunc) http.HandlerFunc
+	}
+
+	maxClients struct {
+		pool    chan struct{}
+		timeout time.Duration
+	}
+)
+
+const defaultRequestDeadline = time.Second * 30
+
+func NewMaxClientsMiddleware(count int, timeout time.Duration) MaxClients {
+	if timeout <= 0 {
+		timeout = defaultRequestDeadline
+	}
+
+	return &maxClients{
+		pool:    make(chan struct{}, count),
+		timeout: timeout,
+	}
+}
+
+func (m *maxClients) Handle(f http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		if m.pool == nil {
+			f.ServeHTTP(w, r)
+			return
+		}
+
+		deadline := time.NewTimer(m.timeout)
+		defer deadline.Stop()
+
+		select {
+		case m.pool <- struct{}{}:
+			defer func() { <-m.pool }()
+			f.ServeHTTP(w, r)
+		case <-deadline.C:
+			// Send a http timeout message
+			WriteErrorResponse(r.Context(), w,
+				errorCodes.ToAPIErr(ErrOperationMaxedOut),
+				r.URL)
+			return
+		case <-r.Context().Done():
+			return
+		}
+	}
+}