Improve ticker channels: make them cross-platform and compatible with test context

[elizarov]

See #327 and #297 for background. We need to support more time-related methods in Delay interface (that can be optionally implemented by CoroutineDispatcher) so that time-depended operators like ticker can be implemented in a time-source-neutral way and will work on all platforms and under purely fictional (test) time source.

[nvbln]

Hi,

Just wondering, how is this issue progressing?