Updates to slack-adapter

[tracyboehrer (tracyboehrer)]

Fixes #1548

There have been some some updates to the DotNet slack-adapter, and this PR includes most of those changes.

[Omid Jafari (omid-jf)]

I noticed in my tests that channel_id of a ConversationReference object (which is always "slack") is not the same as the channel ID that chat.delete API requires.
Therefore, I believe in line 137 of slack_adapter.py in this PR, channel=reference.channel_id should be replaced by channel=reference.conversation.id
Moreover, since the current code of slack_helper.py does not set the timestamp property of activities, exception on line 134 of slack_adapter.py in this PR is always raised. There is a fix for this in PR #1605 (a conversion from datetime to epoch is required if using that PR) or ts=context.activity.timestamp in line 137 of slack_adapter.py in this PR can be replaced by ts=reference.activity_id since we already set activity_id to be the timestamp.

[tracyboehrer (tracyboehrer)]

Omid Jafari (@omid-jf) Thanks! I'll take a look and apply the fixes.

[Axel Suárez (axelsrz)]

This is nitpicking but this could be changed to:
self.options = options or SlackAdapterOptions

[Axel Suárez (axelsrz)]

Approved with comments