Improve sync_test.go, add time to SyncNote

[DrJosh9000]

• sync.go: Add time of message to SyncNote, to aid debugging
• various: Set the new Time field
• sync_test.go: Add clarifying comments
• sync_test.go: Clean up some test failure messages
• sync_test.go: Remove named return args on newSyncTest
• sync_test.go: IPv4/v6 distinction not used, so remove arg from newLocalTCPListener
• sync.go: Minor refactor of sync session creation into own method
• sync.go: Cleanup of infinite loop + one-case select -> range over ticker

This upstreams some work by @baptr.