[C++] Use atomic backpressure counter

There are a few places in the code (sink_node.cc, source_node.cc, file_base.cc) where the backpressure counter is of type `int32_t`. This prevents `ExecNode::Pause(...)` and  `ExecNode::Resume(...)` from being thread-safe. The proposal is to make these backpressure counters be of type `std::atomic<int32_t>`.