[CACHE] Use a smaller expected size when serializing query results

BytesStreamOutput allows to pass the expected size but by default uses
BigArrays.PAGE_SIZE_IN_BYTES which is 16k. A common cached result ie.
a date histogram with 3 buckets is ~100byte so 16k might be very wasteful
since we don't shrink to the actual size once we are done serializing.
By passing 512 as the expected size we will resize the byte array in the stream
slowly until we hit the page size and don't waste too much memory for small query
results.

src/main/java/org/elasticsearch/indices/cache/query/IndicesQueryCache.java

@@ -264,7 +264,16 @@ public class IndicesQueryCache extends AbstractComponent implements RemovalListe
         @Override
         public Value call() throws Exception {
             queryPhase.execute(context);
-            try (BytesStreamOutput out = new BytesStreamOutput()) {
+
+            /* BytesStreamOutput allows to pass the expected size but by default uses
+             * BigArrays.PAGE_SIZE_IN_BYTES which is 16k. A common cached result ie.
+             * a date histogram with 3 buckets is ~100byte so 16k might be very wasteful
+             * since we don't shrink to the actual size once we are done serializing.
+             * By passing 512 as the expected size we will resize the byte array in the stream
+             * slowly until we hit the page size and don't waste too much memory for small query
+             * results.*/
+            final int expectedSizeInBytes = 512;
+            try (BytesStreamOutput out = new BytesStreamOutput(expectedSizeInBytes)) {
                 context.queryResult().writeToNoId(out);
                 // for now, keep the paged data structure, which might have unused bytes to fill a page, but better to keep
                 // the memory properly paged instead of having varied sized bytes