IMPALA-330: Use R/W lock to protect path-handlers in webserver

Change-Id: I62027e015bbb4e1e3b4a8a6ca6eaa612b3a9fc57
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1650
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins