time: Add Until helper function

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>

src/time/time.go

@@ -640,6 +640,12 @@ func Since(t Time) Duration {
 	return Now().Sub(t)
 }
 
+// Until returns the duration until t.
+// It is shorthand for t.Sub(time.Now()).
+func Until(t Time) Duration {
+	return t.Sub(Now())
+}
+
 // AddDate returns the time corresponding to adding the
 // given number of years, months, and days to t.
 // For example, AddDate(-1, 2, 3) applied to January 1, 2011