event: deprecate TypeMux and related types

The Subscription type is gone, all uses are replaced by
*TypeMuxSubscription. This change is prep-work for the
introduction of the new Subscription type in a later commit.

   gorename -from '"github.com/ethereum/go-ethereum/event"::Event' -to TypeMuxEvent
   gorename -from '"github.com/ethereum/go-ethereum/event"::muxsub' -to TypeMuxSubscription
   gofmt -w -r 'Subscription -> *TypeMuxSubscription' ./event/*.go
   find . -name '*.go' -and -not -regex '\./vendor/.*' \| xargs gofmt -w -r 'event.Subscription -> *event.TypeMuxSubscription'

light/txpool.go

@@ -47,7 +47,7 @@ type TxPool struct {
 	signer   types.Signer
 	quit     chan bool
 	eventMux *event.TypeMux
-	events   event.Subscription
+	events   *event.TypeMuxSubscription
 	mu       sync.RWMutex
 	chain    *LightChain
 	odr      OdrBackend