Fix overflow panic when using Duration::MAX for timeout

[rumpuslabs]

I tried to get away with passing Duration::MAX as a timeout to a function that ultimately called Receiver::recv_timeout, and got a panic.

The docs say

For cross-platform code, you can comfortably use durations of up to around one hundred years.

This PR avoids the panic by approximating durations over 100 years (or however far away your platform's maximum Instant is) with "never".

[zesterer]

Thanks! This seems like a fairly unambiguous improvement.