Fix `panic: "POST /minio/peer/v21/signalservice?signal=2": sync: WaitGroup is reused before previous Wait has returned`
Log entries already on the channel would cause `logEntry` to increment the
waitgroup when sending messages, after Cancel has been called.
Instead of tracking every single message, just check the send goroutine. Faster
and safe, since it will not decrement until the channel is closed.
Regression from #14289
Klaus Post
Anis Elleuch
Harshavardhana
Shireesh Anjal