ASTERIXDB-1157, ASTERIXDB-1051: Pushdown limit

- Limit Pushdown into an Order (ExternalSort) operator.
- CopyLimitDownRule doesn't copy LIMIT through UNNESTMAP operator.

Change-Id: I49fb5f38fe8eb4b4419e596a03e2187939d9fd2e
Reviewed-on: https://asterix-gerrit.ics.uci.edu/616
Reviewed-by: Yingyi Bu <buyingyi@gmail.com>
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>