[SPARK-3106] Fix the race condition issue about Connection and ConnectionManager

I understand registering DEFAULT_INTEREST (OP_READ) is to detect closing connection by remote host.
But, once blocked by channel.read() in SendingConnection#read, DEFAULT_INTEREST is not needed.

In addition, because SendingConnection is never unregistered OP_READ, 2 threads on the same SendingConnection should be active and during one of the thread cancels its key, another thread can evaluate key.isValid in ConnectionManager#run.