VStreamer: Schema versioning

[rohit-nayak-ps]

No description provided.

[sougou]

Some feedback life-cycle management, which I forgot to ramp you up on.
Also, I think we can simplify historian's code as mentioned in the comment.

[sougou]

Nice work. I added some nit-picks.

[sougou]

Since sch doesn't escape out of this function, it's better to directly assign sch.Tables. No need to copy.

[rohit-nayak-ps]

sch.Tables is a list, converting to a map.

[indera-shsp]

This implements rfc - #6300

From a Slack discussion -- Rohit mentioned

If you stream from the primary the vstream will handle DDLs correctly. 
You should run the tracker on the primary. If you are streaming realtime then you don't need the tracker 
(the stream will pause while the latest schema is loaded), 
but if streams are run for older events then the tracker will provide the correct schema for the older events.

You should run the tracker on the primary

@rohit-nayak-ps What does primary mean here? My interpretation is that we should enable these two flags

-watch_replication_stream and -track_schema_versions

on all replica tablets since any of them can become primary.