Preparations for removing BaseDendrite (#3016)

Preparations to actually remove/replace `BaseDendrite`.
Quite a few changes:
- SyncAPI accepts an `fulltext.Indexer` interface (fulltext is removed
from `BaseDendrite`)
- Caches are removed from `BaseDendrite`
- Introduces a `Router` struct (likely to change)
  - also fixes #2903
- Introduces a `sqlutil.ConnectionManager`, which should remove
`base.DatabaseConnection` later on
- probably more

internal/httputil/routing_test.go

@@ -0,0 +1,38 @@
+package httputil
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"path/filepath"
+	"testing"
+)
+
+func TestRoutersError(t *testing.T) {
+	r := NewRouters()
+
+	// not found test
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, filepath.Join(PublicFederationPathPrefix, "doesnotexist"), nil)
+	r.Federation.ServeHTTP(rec, req)
+	if rec.Code != http.StatusNotFound {
+		t.Fatalf("unexpected status code: %d - %s", rec.Code, rec.Body.String())
+	}
+	if ct := rec.Header().Get("Content-Type"); ct != "application/json" {
+		t.Fatalf("unexpected content-type: %s", ct)
+	}
+
+	// not allowed test
+	r.DendriteAdmin.
+		Handle("/test", http.HandlerFunc(func(writer http.ResponseWriter, request *http.Request) {})).
+		Methods(http.MethodPost)
+
+	rec = httptest.NewRecorder()
+	req = httptest.NewRequest(http.MethodGet, filepath.Join(DendriteAdminPathPrefix, "test"), nil)
+	r.DendriteAdmin.ServeHTTP(rec, req)
+	if rec.Code != http.StatusMethodNotAllowed {
+		t.Fatalf("unexpected status code: %d - %s", rec.Code, rec.Body.String())
+	}
+	if ct := rec.Header().Get("Content-Type"); ct != "application/json" {
+		t.Fatalf("unexpected content-type: %s", ct)
+	}
+}