Deprecate Silence attribute and Fix multiple terraform runs

[nmuesch]

This PR looks to accomplish a few things surrounding the silence attribute of monitor resources.

• Add a deprecation notice that we'll stop supporting the silenced param soon. (In a couple Major versions of the Provider as outlined by the best practices doc - https://www.terraform.io/docs/extend/best-practices/deprecations.html)
• Add the ability to specify a -1 on any scope of the silenced attribute to unmute that scope. This will make a separate API call to unmute those scopes (Only on updates, newly created monitors shouldn't be muted :) )
• Fix an issue where running terraform apply would detect a diff on the silence scope if the timestamp were in the past. This is due to the Datadog Monitor API not returning scopes with timestamps in the past in the API response. So the monitor resource will detect this and leave the state unchanged.
• Add some tests 🎉

[bkabrda]

So overall this is a real improvement, except one scenario that I pointed in one of the comments. Whenever you fix that, please consider adding some tests for this, since it's somewhat unintuitive (at least for me) how this works.

[gzussa]

👍

[bkabrda]

LGTM now, nice job! 👍

I only have one minor nit in terms of test names - if you could name them TestAccDatadogMonitor_Silenced*, it would be easier to run all tests related to silenced with -run TestAccDatadogMonitor_Silenced when running go test. I think it would be more practical, but I don't mean to block this PR getting merged on it.

Also, please squash the commits that you did to not pollute project history. Thanks!

[nmuesch]

Good idea, just changed the test function names based on your suggestion. I'll do a squash and merge if thats alright with you.

[bkabrda]

Yup, go ahead with squashing and merging 👍

[gzussa]

Great stuff :)