ASTERIXDB-1205: fix union queries.

Fixed the AqlExpressionToPlanTranslator for union expression;
Eliminate shared operator references in translated logical plans;
Removed IntroduceUnionRule;
Fixed tuple source for SQL++ join clause;
Added regression tests.

Change-Id: Ib79a899ec4b35ece6f8f9f59b80126fed0c48851
Reviewed-on: https://asterix-gerrit.ics.uci.edu/629
Reviewed-by: Till Westmann <tillw@apache.org>
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
diff --git a/asterix-app/src/test/resources/optimizerts/results/union/query-ASTERIXDB-1205.plan b/asterix-app/src/test/resources/optimizerts/results/union/query-ASTERIXDB-1205.plan
new file mode 100644
index 0000000..115b5b6
--- /dev/null
+++ b/asterix-app/src/test/resources/optimizerts/results/union/query-ASTERIXDB-1205.plan
@@ -0,0 +1,19 @@
+-- DISTRIBUTE_RESULT  |UNPARTITIONED|
+  -- ONE_TO_ONE_EXCHANGE  |UNPARTITIONED|
+    -- STREAM_PROJECT  |UNPARTITIONED|
+      -- ONE_TO_ONE_EXCHANGE  |UNPARTITIONED|
+        -- STABLE_SORT [$$20(ASC), $$21(ASC)]  |UNPARTITIONED|
+          -- ONE_TO_ONE_EXCHANGE  |UNPARTITIONED|
+            -- UNION_ALL  |UNPARTITIONED|
+              -- ONE_TO_ONE_EXCHANGE  |UNPARTITIONED|
+                -- ASSIGN  |UNPARTITIONED|
+                  -- STREAM_PROJECT  |UNPARTITIONED|
+                    -- UNNEST  |UNPARTITIONED|
+                      -- ASSIGN  |UNPARTITIONED|
+                        -- EMPTY_TUPLE_SOURCE  |UNPARTITIONED|
+              -- ONE_TO_ONE_EXCHANGE  |UNPARTITIONED|
+                -- ASSIGN  |UNPARTITIONED|
+                  -- STREAM_PROJECT  |UNPARTITIONED|
+                    -- UNNEST  |UNPARTITIONED|
+                      -- ASSIGN  |UNPARTITIONED|
+                        -- EMPTY_TUPLE_SOURCE  |UNPARTITIONED|