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
```
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