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/fulltext/bleve_wasm.go

@@ -15,8 +15,9 @@
 package fulltext
 
 import (
-	"github.com/matrix-org/dendrite/setup/config"
 	"time"
+
+	"github.com/matrix-org/dendrite/setup/config"
 )
 
 type Search struct{}
@@ -28,6 +29,13 @@ type IndexElement struct {
 	StreamPosition int64
 }
 
+type Indexer interface {
+	Index(elements ...IndexElement) error
+	Delete(eventID string) error
+	Search(term string, roomIDs, keys []string, limit, from int, orderByStreamPos bool) (SearchResult, error)
+	Close() error
+}
+
 type SearchResult struct {
 	Status   interface{}   `json:"status"`
 	Request  *interface{}  `json:"request"`
@@ -48,7 +56,7 @@ func (f *Search) Close() error {
 	return nil
 }
 
-func (f *Search) Index(e IndexElement) error {
+func (f *Search) Index(e ...IndexElement) error {
 	return nil
 }