message: Correctly update buffer manager memory when block is destroyed
diff: @@ -31,6 +31,11 @@ BlockHandle::BlockHandle(BufferManager &manager_p, block_id_t block_id_p, unique
  BlockHandle::~BlockHandle() {
  // no references remain to this block: erase
  + if (state == BlockState::BLOCK_LOADED) {
  + // the block is still loaded in memory: erase it
  + buffer.reset();
  + manager.current_memory -= memory_usage;
  + }
  manager.UnregisterBlock(block_id, can_destroy);
  }
  