CLEANUP: pools: rename __pool_free() to pool_put_to_shared_cache()

Now the multi-level cache becomes more visible:

    pool_get_from_local_cache()
    pool_put_to_local_cache()
    pool_get_from_shared_cache()
    pool_put_to_shared_cache()
diff --git a/src/pool.c b/src/pool.c
index d1e48fd..db5d931 100644
--- a/src/pool.c
+++ b/src/pool.c
@@ -141,7 +141,7 @@
 		ph->count--;
 		pool_cache_count--;
 		pool_cache_bytes -= pool->size;
-		__pool_free(pool, item);
+		pool_put_to_shared_cache(pool, item);
 	} while (pool_cache_bytes > CONFIG_HAP_POOL_CACHE_SIZE * 7 / 8);
 }
 #endif