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/caching/impl_ristretto.go

@@ -46,6 +46,11 @@ const (
 	eventStateKeyNIDCache
 )
 
+const (
+	DisableMetrics = false
+	EnableMetrics  = true
+)
+
 func NewRistrettoCache(maxCost config.DataUnit, maxAge time.Duration, enablePrometheus bool) *Caches {
 	cache, err := ristretto.NewCache(&ristretto.Config{
 		NumCounters: int64((maxCost / 1024) * 10), // 10 counters per 1KB data, affects bloom filter size