ENG-4676: eliminate OrderBy plan node for JOIN + ORDER BY when ordering by indexed columns; push down LIMIT node for JOIN node with sort direction.
ENG-4676:
1. remove LIMIT push down change from previous commit, this will be implemented in a separate ticket;
2. put sortDirection in AbstractJoinPlanNode in order to ignore ORDER BY node for both NLIJ and NLJ
3. add more junit tests and more comments.
Merge branch 'master' into ENG-4676

Conflicts:
	tests/frontend/org/voltdb/regressionsuites/TestOrderBySuite.java
ENG-4676: cleanup for change of LIMIT; override resolveSortDirection in NLIJ
Merge branch 'master' into ENG-4676
Merge branch 'fix-indexcount' into ENG-4676
Merge branch 'master' into ENG-4676
Merge recent master and resolve conflicts
Clean up the test cases by using the implemented functions
Merge pull request #700 from VoltDB/ENG-4676

ENG-4676: eliminate OrderBy plan node for JOIN + ORDER BY when ordering ...