The master decodes a TTL per volume in every heartbeat and keeps it for the
volume's lifetime, so a cluster using TTLs carries one two-byte object per
volume replica where at most 256 counts times 7 units exist. Share them, and
decode the uint32 form directly instead of staging it through a byte slice.
Clusters that set no TTL are unaffected; that path already returned the shared
EMPTY_TTL.
BenchmarkSyncDataNodeRegistration/100000Volumes, volumes carrying a ttl
600600 allocs/op -> 500597 allocs/op