Selector completes receives on a connection before completing sends if there are errors

Nobody has claimed this yet.

Just noticed during some testing that the ambry selector can complete receive on a connection id when the send is not complete. A quick look at the code indicates that the interest ops are only set to READ when the send is completed, so not sure how this happens. This seems like a bug and needs to be investigated and fixed.

This happened when there were timeout errors and such happening, but interestingly, did not notice any error on the said connection (if not it would not have been able to complete the receive, I think)

Will try to add more details.

```
2016/06/27 17:08:14.590 INFO [NetworkClient] [RequestResponseHandlerThread-1] [ambry-frontend-nb] [] ***Connection checkout succeeded for lva
1-app2039.stg.linkedin.com:Port[15088:PLAINTEXT] with connectionId 0.0.0.0:-1-10.136.152.27:15088_9
2016/06/27 17:08:14.710 INFO [NetworkClient] [RequestResponseHandlerThread-1] [ambry-frontend-nb] [] ***Receive completed for connectionId 0.0.0.0:-1-10.136.152.27:15088_9 and checking in the connection back to connection tracker

...

2016/06/27 17:08:16.591 INFO [NetworkClient] [RequestResponseHandlerThread-1] [ambry-frontend-nb] [] ***Connection checkout succeeded for lva1-app2039.stg.linkedin.com:Port[15088:PLAINTEXT] with connectionId 0.0.0.0:-1-10.136.152.27:15088_9
2016/06/27 17:08:16.591 INFO [Selector] [RequestResponseHandlerThread-1] [ambry-frontend-nb] [] ***Setting NetworkSend threw an exception for connection id: 0.0.0.0:-1-10.136.152.27:15088_9
2016/06/27 17:08:16.592 ERROR [NonBlockingRouter] [RequestResponseHandlerThread-1] [ambry-frontend-nb] [] Aborting, as requestResponseHandlerThread received an unexpected error:
java.lang.IllegalStateException: Attempt to begin a networkSend operation with prior networkSend operation still in progress.
at com.github.ambry.network.Transmission.setNetworkSend(Transmission.java:65)
```

Start by tracing the selector behavior around the logged `Selector` exception and `Transmission.java:65`, then follow how `NonBlockingRouter` handles send and receive completion during timeout errors. Reproduce the ordering problem if possible and add coverage for a connection whose receive completes before its send; done means the invalid overlap no longer occurs and the relevant test passes.

Written by the indexing model from the issue text.