Replace endpoint events/count by documentCount

We presume that contDocument we will retrieve the data more efficiently that the way is done today with find({}).length

Mongoose supports countDocument https://mongoosejs.com/docs/api.html#model_Model.countDocuments

Current performance is:

Data taken from: https://gitlab.esrf.fr/demariaa/performancetests/-/tree/master/logbook/QueryAndCount/tests/localhost_icatplus_2