Add basic RPC instrumentation

This patch adds the /rpcz endpoint, which shows aggregate statistics for
all rpc invocations

Change-Id: I7b32c5ae09360cea61d4a79d7e580ac7048b19d5
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2909
Tested-by: jenkins
Reviewed-by: Henry Robinson <henry@cloudera.com>