Allow to pull a set of packages per request

Currently, the sync/reverse agent pull one package per request.
This protocol is really simple and it is already possible to decrease the polling interval (via the trigger) to increase the sync throughput.
However, establishing a request has a cost which is applied to every packages.

In order to improve the throughput when processing the queue, we could avoid establishing most of the request, by allowing to pull more than one package per request.
In order to allow for some flow control, we could let the receiving side define how many packages could be fetched (similarly to how TCP works).

Allowing to pull more than more package per request adds more complexity to the code, so we may first build a PoC to demonstrate if the improved throughput is worth the added complexity.

cc teofili, simone.tripodi