[Reducing DB interactions at mongoBackend] Fill ONCHANGE notifications information from thread memory instead of entityQuery()

[fgalan]

As in version 0.20.0, the "filling" of ONCHANGE notification is done using entitiesQuery() operation, which involves a new query to DB (i.e. suboptimal, given that that information is already in the thread memory processing the updateContext).

This issue is about changing the way it works, so avoiiding the additional query to DB.

[fgalan]

Not sure at the end if all information is already known, e.g. in the case of subscriptions with geo-location scopes associated. We need entitiesQuery() to solve that notifications.

[fgalan]

After analyzing this a little bit, there is a couple of "spin off" issues also related with reducing the number of DB interactions in mongoBackend:

• [Reducing DB interactions at mongoBackend] Use csubs cache also for not pattern subscriptions with just one lookup operation #1475
• [Reducing DB interactions at mongoBackend] Avoid update csub when notifications are sent #1476

[fgalan]

Implemented in PR #1483