refactor how session tickets are sent

Previously, RunHandshake() would send the session tickets. Now, the
session initiates the sending of the session ticket. This simplifies the
setup a bit, and it will make it possible to include the RTT estimate in
the session ticket without accessing the RTTStats concurrently.

internal/handshake/interface.go

@@ -72,6 +72,7 @@ type CryptoSetup interface {
 	RunHandshake()
 	io.Closer
 	ChangeConnectionID(protocol.ConnectionID)
+	GetSessionTicket() ([]byte, error)
 
 	HandleMessage([]byte, protocol.EncryptionLevel) bool
 	SetLargest1RTTAcked(protocol.PacketNumber)