Add crontab timer function to run periodic job

[lqhuang]

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Description

I'm using mode as base framework to build some applications, but I found missing some features about Service.timer and Service.task.

So I try to add a crontab based timer into mode, and the major implementation is ported from faust code base.

It won't be better you could help to review this PR. If there are some problems, I can follow your instruction to improve them.

Regards.

Lanqing

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Please describe your pull request.

NOTE: All patches should be made against master, not a maintenance branch like
3.1, 2.5, etc. That is unless the bug is already fixed in master, but not in
that version series.

If it fixes a bug or resolves a feature request,
be sure to link to that issue via (Fixes #4412) for example.

[max-k]

IMO, porting back crontab from Faust to Mode is a good idea but if we do that, we'll also have to update Faust to make it consume crontab functionality from here instead of maintaining two similar features in two different projects.

Unfortunately, @patkivikram doesn't seem willing to accept merge requests on Faust project.

We opened a very simple MR with a teammate before Christmas and we still didn't get any comment at this time.

Moreover, this project lack of a communication system like Gitter so it's a bit difficult to open a discussion.

@patkivikram : Could you give us your opinion about that ? Thank you very much.

[patkivikram]

@max-k which PR are you referring to in faust-streaming? We are accepting and merging PR's on a regular basis

[lqhuang]

OK, then I'm willing to do the job that update codes in faust after PR has been merged and released a new version.

[max-k]

@patkivikram Hi. Thank you for your answer. I talk about This one : faust-streaming/faust#66
@lqhuang Thank you very much. It will be great if you can do that.

[lqhuang]

@max-k Thank you for merging my PR :) Future more, when would a new version be released? That will be helpful for me to update faust codes.

[max-k]

Hi @lqhuang
I'll try to automate the release process during next week if I have enough time.
Otherwise I'll do a manual release at the end of the week.