IMPALA-3086: Fix unsynchronized access to DataStreaMgr::pending_rendezvous_
IMPALA-3086: Fix unsynchronized access to DataStreaMgr::pending_rendezvous_

Change-Id: I80dfbb192271152f04cfc8dc7ccced500d5dc6ae
Reviewed-on: http://gerrit.cloudera.org:8080/2317
Tested-by: Internal Jenkins
Reviewed-by: Silvius Rus <f7ea5851fd59d0434c2e152e6169f61c38e0d175@cloudera.com>

cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala