Tests at very high scale indicates that the iteration
of all connections during websocket broadcast starts
to become a bottleneck.
To optimize this, we move the channelMember cache from
inside web_conn.go to the hubConnectionIndex.
This involves adding a new map keyed by the channelID
and containing all webConns where the user is a member
of that channel. Subsequently, a new method needed to
be added to invalidate the cache which previously
used to happen in web_conn.
And as a last step, we remove the cache from web_conn
to reduce SQL queries to the DB.
https://mattermost.atlassian.net/browse/MM-61130
```release-note
NONE
```
It was a good decision in hindsight to keep the public module as 0.x
because this would have been a breaking change again.
https://mattermost.atlassian.net/browse/MM-53032
```release-note
Changed the Go module path from github.com/mattermost/mattermost-server/server/v8 to github.com/mattermost/mattermost/server/v8.
For the public facing module, it's path is also changed from github.com/mattermost/mattermost-server/server/public to github.com/mattermost/mattermost/server/public
```
