Methods with timeout should not crash on negative time

[elizarov]

Methods like delay, withTimeout, onTimeout all require(time >= 0). This leads to error-prone time-based code. See gist for #306 for example: https://gist.github.com/elizarov/34150213209158d0dce0db62d9c0a20f

This code in this gist happens to work if this line is removed, but it would still wrong without this this line (it will subtly main sometimes):

if (remainingNanos <= 0) break // this window is over

We should remove require(time >= 0) from all time-based methods. They should assume that they should not delay (or assume imediate timeout) when negative time is passed.

[qwwdfsad]

CompletableFuture immediately throws TimeoutException if a timeout is not positive.
Rx, Rx2, and Reactor throw TimeoutException if a timeout is not positive, though timeout task is properly scheduled. Negative delay treated as no delay everywhere, but with Delay task scheduling