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

setup/jetstream/nats.go

@@ -20,6 +20,8 @@ import (
 
 type NATSInstance struct {
 	*natsserver.Server
+	nc *natsclient.Conn
+	js natsclient.JetStreamContext
 }
 
 var natsLock sync.Mutex
@@ -69,11 +71,18 @@ func (s *NATSInstance) Prepare(process *process.ProcessContext, cfg *config.JetS
 	if !s.ReadyForConnections(time.Second * 10) {
 		logrus.Fatalln("NATS did not start in time")
 	}
+	// reuse existing connections
+	if s.nc != nil {
+		return s.js, s.nc
+	}
 	nc, err := natsclient.Connect("", natsclient.InProcessServer(s))
 	if err != nil {
 		logrus.Fatalln("Failed to create NATS client")
 	}
-	return setupNATS(process, cfg, nc)
+	js, _ := setupNATS(process, cfg, nc)
+	s.js = js
+	s.nc = nc
+	return js, nc
 }
 
 func setupNATS(process *process.ProcessContext, cfg *config.JetStream, nc *natsclient.Conn) (natsclient.JetStreamContext, *natsclient.Conn) {