Refactor roomserver/internal - split perform stuff out (#1380)

- New package `perform` which contains all `Perform` functions
- New package `helpers` which contains helper functions used by both
  perform and query/input functions.
- Perform invite/leave have no idea how to `WriteOutputEvents` and this
  is now returned from `PerformInvite` or `PerformLeave` respectively.

Still to do:
 - RSAPI is fed into the inviter/joiner/leaver - this introduces circular
   logic so will need to be removed.
 - Put query operations in a `query` package.
 - Put input operations (and output) in an `input` package.
 - Factor out helper functions as much as possible, possibly rejigging the
   storage layer in the process.

roomserver/internal/input.go

@@ -23,17 +23,8 @@ import (
 	"github.com/Shopify/sarama"
 	"github.com/matrix-org/dendrite/roomserver/api"
 	log "github.com/sirupsen/logrus"
-
-	fsAPI "github.com/matrix-org/dendrite/federationsender/api"
 )
 
-// SetFederationSenderInputAPI passes in a federation sender input API reference
-// so that we can avoid the chicken-and-egg problem of both the roomserver input API
-// and the federation sender input API being interdependent.
-func (r *RoomserverInternalAPI) SetFederationSenderAPI(fsAPI fsAPI.FederationSenderInternalAPI) {
-	r.fsAPI = fsAPI
-}
-
 // WriteOutputEvents implements OutputRoomEventWriter
 func (r *RoomserverInternalAPI) WriteOutputEvents(roomID string, updates []api.OutputEvent) error {
 	messages := make([]*sarama.ProducerMessage, len(updates))