rpc: Print response correctly under VLOG(4)
rpc: Print response correctly under VLOG(4)

This fixes a bug where we print the request instead of the response
under VLOG(4) where we intended to print the response. Example output
after the fix below (from rpc_stub-test -vmodule=rpc_context=4):

[ RUN      ] RpcStubTest.TestCallbackClearedAfterRunning
W0430 15:16:06.476577 15726 reactor.cc:200] Client call kudu.rpc_test.CalculatorService.Add has no timeout set for connection id: {remote=0.0.0.0:46438, user_credentials={real_user=mpercy, eff_user=}}
I0430 15:16:06.478302 15723 rpc_context.cc:29] kudu.rpc_test.CalculatorService: Received RPC request for Call kudu.rpc_test.CalculatorService.Add from 127.0.0.1:60879 (request call id 1):
x: 10
y: 20
I0430 15:16:06.478412 15723 rpc_context.cc:38] kudu.rpc_test.CalculatorService: Sending RPC success response for Call kudu.rpc_test.CalculatorService.Add from 127.0.0.1:60879 (request call id 1):
result: 30

Change-Id: Ic04e5e9aec734ff005fea7c8b9d0d64f0a9e6de2
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/6549
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>
Tested-by: jenkins

helifu/kudu,helifu/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,InspurUSA/kudu,andrwng/kudu,andrwng/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,andrwng/kudu,InspurUSA/kudu,helifu/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,andrwng/kudu,andrwng/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,andrwng/kudu,InspurUSA/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,cloudera/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu