Rename allocator to allocator_t

src/blockstore/blockstore_impl.h

@@ -279,7 +279,7 @@ class blockstore_impl_t
     std::vector<obj_ver_id> unsynced_big_writes, unsynced_small_writes;
     int unsynced_big_write_count = 0, unstable_unsynced = 0;
     int unsynced_queued_ops = 0;
-    allocator *data_alloc = NULL;
+    allocator_t *data_alloc = NULL;
     uint64_t used_blocks = 0;
     uint8_t *zero_object;