MPMCQueue blockingRead() doesn't return even with a subsequent write()

An example to demonstrate it:

```
#include
#include
#include "folly/MPMCQueue.h"

int main() {
folly::MPMCQueue q(1024);
std::thread t([&q](){
int i;
std::cout << q.size() << std::endl;
q.blockingRead(i); // never wake up
std::cout << q.size() << std::endl;
std::cout << i << std::endl;
});

sleep(1);
std::cout << q.size() << std::endl;
q.write(333);
std::cout << q.size() << std::endl;
while(true);
}
```

Start by running the reproducer in the issue and reading folly/MPMCQueue.h, focusing on blockingRead() and the write() path that should wake it. Done means a waiting blockingRead() returns after write(333) and receives 333; add or update a focused regression test if the repository provides a suitable queue test location.

Written by the indexing model from the issue text.