Make it possible to send several service calls in one HTTP request

[balat]

Generating pages from client side, especially with reactive pages (using lots of ow_spinners), may require lots of server requests. I suggest we coalesce them, at Eliom level (so that the programmer does not have to think about this). May be: before sending the request, we put it in a stack, call Lwt.yield, and then send all the stacked requests together.