Only add to timeoutExchanges if exchange was deleted
Only add to timeoutExchanges if exchange was deleted

In a race between timing out an exchange and removing an exchange, if
the remove wins, the timeout would add the exchange to timeoutExchanges
anyway, which could lead to sendChRefs.Done() being called twice.

We should only add an exchange to timeoutExchanges if the exchange was
found in exchanges.

uber/tchannel-go,uber/tchannel-go