Fix race around read and write deadlines in Stream

[filipochnik]

I found this race condition when running my code using yamux with -race.

Since the test suite fails when run with -race I don't think I can write a meaningful test here.

[jefferai]

I think it is likely to be faster to use an atomic.Value here, since it is only being read in one place and written in another.

[filipochnik]

Sure, I changed it to atomic.Value :)

[jefferai]

Thanks!