// Subroute will be 32 bytes long and added before any CRC.
	FlagSubroute
)

// This struct cannot be changed and retain backwards compatibility.
// If changed, endpoint version must be bumped.
//
//msgp:tuple message
type message struct {
	MuxID      uint64    // Mux to receive message if any.
	Seq        uint32    // Sequence number.
	DeadlineMS uint32    // If non-zero, milliseconds until deadline (max 1193h2m47.295s, ~49 days)

| ...header_n     | msgp bin array              | Header of last version
| ...metadata_n   | msgp bin array              | Metadata of last version

Each header contains a mspg array (tuple) encoded object:

xlHeaderVersion version == 1:

```
//msgp:tuple xlMetaV2VersionHeader
type xlMetaV2VersionHeader struct {
 VersionID [16]byte  // Version UUID, raw.
 ModTime   int64     // Unix nanoseconds.