perf: process large queries in chunks

Signed-off-by: Miroslav Bajtoš <mbajtoss@gmail.com>

lib/dao.js

@@ -1667,7 +1667,30 @@ DataAccessObject.find = function find(query, options, cb) {
       cb(err, results);
     }
 
-    processQueryResults(data, done);
+    // FIXME(bajtos) read this value from Model settings
+    const CHUNK_SIZE = 500;
+    if (data.length <= CHUNK_SIZE) {
+      processQueryResults(data, done);
+    } else {
+      const chunks = [];
+      while (data.length)
+        chunks.push(data.splice(0, CHUNK_SIZE));
+      async.mapSeries(chunks, function processChunkOfQueryResults(data, next) {
+        let sync = true;
+        processQueryResults(data, (err, result) => {
+          if (sync) {
+            process.nextTick(next, err, result);
+          } else {
+            next(err, result);
+          }
+        });
+        sync = false;
+      }, (err, resultChunks) => {
+        if (err) return done(err);
+        const result = [].concat(...resultChunks);
+        done(null, result);
+      });
+    }
   };
 
   function processQueryResults(data, cb) {