[performance] reduce InboxForward->Create calls by partially implementing Exists() (#3647)

* alphabetical reordering

* keep a cache of activity IDs we have handled creates for

* reduce number of inbox forwarding create calls by partially implementing Exists()

* increase cache size, since all we're storing is string keys

internal/federation/federator.go

@@ -33,7 +33,7 @@ import (
 var _ interface {
 	pub.CommonBehavior
 	pub.FederatingProtocol
-} = &Federator{}
+} = (*Federator)(nil)
 
 type Federator struct {
 	db                  db.DB