[cluster] fill sendbuffer better

use a counter to track available elements.

Some elo gain, on 4 ranks:

Score of old-r4-1t vs new-r4-1t: 422 - 508 - 1694  [0.484] 2624
Elo difference: -11.39 +/- 7.90

src/thread.h

@@ -79,6 +79,7 @@ public:
   struct {
       Mutex mutex;
       Cluster::TTSendBuffer<Cluster::TTSendBufferSize> buffer = {};
+      size_t counter = 0;
   } ttBuffer;
 #endif
 };