add bytesRead to FileStream

[Uzlopak]

Inspired by mscdex/busboy#174 by @ahuigo

FileStream will be similar to FS.ReadStream and provide the attribute bytesRead.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[kibertoad]

Before: Mean time for fastify-busboy is 291910.52376131905 nanoseconds
After: Mean time for fastify-busboy is 295153.5812366501 nanoseconds

So approximately 1% loss of performance, which is likely to be within the margin of error.

Of course, it is still faster than the original:
Mean time for busboy is 366253.787385089 nanoseconds

@Uzlopak Thoughts? Personally I think it's acceptable if there is benefit to having this data exposed.

[Uzlopak]

On second thought I think this is not the best solution. I reread the code and I think we just need to assign nsize in that branch and the branch above when we truncate the file. There is I think no need for the ternary operation. Will check that later.

[Uzlopak]

Is the mathemal changes made by me correct?

[kibertoad]

Will be back in few hours and will check!

[kibertoad]

wonder if we can have interim state unit test for this

[Uzlopak]

I actually find it prone to a race condition that I first emit the limit event and later set the bytesRead attribute. That's why I moved the truncated attribute change above the emit.

Should we set the bytesRead above the emit? Additional to the bytesRead below the if block.

In worst case bytesRead is set twice it limit is hit, which is imho acceptable.

Wdyt?

[Uzlopak]

made it less race condition prone