increase size of buffer_pool

- This solves an issue where in the worst-case an RLC AM instance would
  block because no new data nor control PDUs could be created to flush
  the buffers, effectivly causing a system stall.
- Happens in RLC AM stress test regularly with higher drop rates.

lib/include/srslte/common/buffer_pool.h

@@ -169,7 +169,7 @@ public:
 
   
 private:  
-  static const int       POOL_SIZE = 2048;
+  static const int       POOL_SIZE = 4096;
   std::stack<buffer_t*>  available;
   std::vector<buffer_t*> used; 
   pthread_mutex_t        mutex;