external-data-source-executor leaking global jni refs

repro:

for i in {1..1000}; do impala-shell.sh -q "select * from functional.alltypes_datasource limit 1;" -B --quiet &> /tmp/a; done

The perm-gen memory slowly grows as reported by the /metrics debug page.

external-data-source-executor was calling JniUtil::LocalToGlobalRef
which adds global refs to a static list and thus never get GC'd.