IMPALA-3500: GetEffectiveUser() segfault using -enable_rm
IMPALA-3500: GetEffectiveUser() segfault using -enable_rm

A previous change to improve admission control broke Llama
integration. The Coordinator's runtime_state has a partial
TQueryCtx, it doesn't have the TSessionState which is used
by RuntimeState::effective_user(), so calling that in the
SimpleScheduler caused the process to crash.

This is easily fixed by using the TSessionState from the
schedule (which is what the Admission Control code was
doing).

Llama integration is no longer supported, so this was only
tested manually.

Change-Id: Ia04b6aec35ae794d7062fd32104f3964f397f00c
Reviewed-on: http://gerrit.cloudera.org:8080/3016
Reviewed-by: Alex Behm <fe1626037acfc2dc542d2aa723a6d14f2464a20c@cloudera.com>
Reviewed-by: Dan Hecht <20ebd99959f0f6e5dcb0fc319b4d8ef53a16b5fb@cloudera.com>
Tested-by: Internal Jenkins

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