uber / tchannel-go / 3314

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bugfix: cancel server context on client tcp close (#635)

**Problem Description:**
When a client crashes, the linux kernel will automatically initiate a tcp_close by sending a FIN request to the server. The server kernel, in response, will the ACK the FIN and move to CLOSE_WAIT, waiting for the app to close the socket. However, tchannel doesn't set the context to done on a connection read error on the server, the conn.Close() won't be called until the context deadline expires. This is bad when tchannel is used for long polling. 

**Solution:**
This patch fixes the problem by marking the server side inbound call context as canceled on a tcp read error. It also marks the msgExchange as expired in the mex, so that it can be subsequently removed. There is already a TODO in the existing code to get rid of the goroutine (on Inbound) per call. This patch does not fix this TODO. 

**Testing:**
Created a unit test that reproduces the mentioned problem. The unit test doesn't use the client channel, instead sends a call directly on the tcp connection.

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