record-level locking...

It would be nice if you can do a record level lock for read/write...

```
fs.flock(fd, 'sh', function(err){
  fs.flockRead(bytesToRead, function(err, buffer){
    //buffer should be the bytes read after an exclusive lock of that record
    // offset at the current position, for the length of the bytes requested to read.
  });
  fs.flockWrite(buffer, function(err){})
});
```
