rpc: Do not close client connection on error response for a timed out request
rpc: Do not close client connection on error response for a timed out request

Client connections were silently dropped when we received an error
response for a hnadler which is no longer there. Log and ignore
instead.

Example log:

  INFO  2016-10-25 11:39:20,010 [shard 0] rpc - client 127.0.0.3: ignoring error response: Semaphore timedout
I

syuu1228/seastar,avikivity/seastar,syuu1228/seastar,dreamsxin/seastar,scylladb/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,dreamsxin/seastar,dreamsxin/seastar,scylladb/seastar,cloudius-systems/seastar,cloudius-systems/seastar