Prefer to throw Unavailable rather than Timeout
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                 Key: CASSANDRA-1988
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-1988
             Project: Cassandra
          Issue Type: Improvement
          Components: API
            Reporter: Stu Hood
             Fix For: 0.8


When a node is unreachable, but is not yet being reported dead by gossip, 
messages are enqueued in the messaging service to be sent when the node becomes 
available again (on the assumption that the connection dropped temporarily).

Higher up in the client layer, before sending messages to other nodes, we check 
that they are alive according to gossip, and fail fast with 
UnavailableException if they are not (CASSANDRA-1803). If we send messages to 
nodes that are not yet being reported dead, the messages sit in queue, and time 
out rather than being sent: this results in the client request failing with a 
TimeoutException.

If we differentiate between messages that were never sent (aka, are still 
queued in the MessagingService at the end of the timeout), and messages that 
were sent but didn't get a response, we can properly throw UnavailableException 
in the former case.

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