Change the mecanism of pagination to handle massive amount of events per investigation

Pagination is currently performed entirely on the client side, while the server sends the whole event list for a given investigation. In the new version, the server sends a slice of events as requested by the client. The client uses this to display the events by pages. The client ask the server for the total number of events in a given investigation.

This work involves work on the server side. See issue icat-plus#16 (closed)