Adds an Until() function that returns the duration until the given time.
This compliments the existing Since() function and makes writing
expressions that have expiration times more readable; for example:

    <-After(time.Until(connExpires)):

Fixes #14595

Change-Id: I87998a924b11d4dad5512e010b29d2da6b123456
Reviewed-on: https://go-review.googlesource.com/20118
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>