Process delta files, create rrdp_loader to centralize rrdp processing.

Process delta files, create rrdp_loader to centralize rrdp processing.

+Parse and process delta files, includes file deleting due to a withdraw as well as the parent dir deletion if the dir is empty.
+Consider that 'publish' elements have an optional 'hash' in some cases.
+Calculate the deltas necessary to process from a notification file, based on the current loaded serial and the last downloaded serial.
+Add handler function to get the last downloaded serial.
+The RRDP loader gets the notification file and takes the decission to process such file (no changes, serial update, session update, or new uri). Its code was at certificate.c, but was rellocated here.
+Remove SLIST from rrdp_objects, as well as some other properties that aren't necessary (lists at delta and at snapshot structs).
+Prepare 'deltas_head' to be referenced from distinct parts.
+Fix serial validation when parsing a 'son' object (e.g. validating a delta that was listed at the update notification file).
+No need to return parsed snapshot and delta, since they are processed asap and no further actions are required with the allocated structs.