Speedup RecyclerBytesStreamOutput.writeString

This method is quite hot in some use-cases because it's used by
most string writing to transport messages. Overriding teh default
implementation for cases where we can write straight to the
page instead of going through an intermediary buffer speeds up
the method by more than 2x, saving lots of cycles, especially
on transport threads.

server/src/main/java/org/elasticsearch/common/io/stream/RecyclerBytesStreamOutput.java

@@ -177,6 +177,35 @@ public void writeWithSizePrefix(Writeable writeable) throws IOException {
         }
     }
 
+    @Override
+    public void writeString(String str) throws IOException {
+        final int currentPageOffset = this.currentPageOffset;
+        final int charCount = str.length();
+        // maximum serialized length is 3 bytes per char + 5 bytes for the longest possible vint
+        if (charCount * 3 + 5 > (pageSize - currentPageOffset)) {
+            super.writeString(str);
+            return;
+        }
+        BytesRef currentPage = pages.get(pageIndex).v();
+        int off = currentPage.offset + currentPageOffset;
+        byte[] buffer = currentPage.bytes;
+        int offset = off + putVInt(buffer, charCount, off);
+        for (int i = 0; i < charCount; i++) {
+            final int c = str.charAt(i);
+            if (c <= 0x007F) {
+                buffer[offset++] = ((byte) c);
+            } else if (c > 0x07FF) {
+                buffer[offset++] = ((byte) (0xE0 | c >> 12 & 0x0F));
+                buffer[offset++] = ((byte) (0x80 | c >> 6 & 0x3F));
+                buffer[offset++] = ((byte) (0x80 | c >> 0 & 0x3F));
+            } else {
+                buffer[offset++] = ((byte) (0xC0 | c >> 6 & 0x1F));
+                buffer[offset++] = ((byte) (0x80 | c >> 0 & 0x3F));
+            }
+        }
+        this.currentPageOffset = offset - currentPage.offset;
+    }
+
     @Override
     public void flush() {
         // nothing to do