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'

core/tx_pool.go

@@ -89,7 +89,7 @@ type TxPool struct {
 	gasLimit     func() *big.Int // The current gas limit function callback
 	minGasPrice  *big.Int
 	eventMux     *event.TypeMux
-	events       event.Subscription
+	events       *event.TypeMuxSubscription
 	localTx      *txSet
 	signer       types.Signer
 	mu           sync.RWMutex