Rollup: Do not batch layer2 operations as they arrived

Prior to this patch, each time a list of layer2 operations was received through the appropriate RPC service, a [Submitted_rollup_operations] layer1 operation was created. This poses two issues. First, the head of the layer1 will change before the resulting operations are sent, and therefore, the fees rate will be outdated. Secondly, it is possible to create just one batch, not several.

This patch changes the behavior of the daemon to allow it to batch layer2 operations once per layer1 change of head.