Set 'root-except-ta' as default, fix bugs deleting RRDP repo files.

Set 'root-except-ta' as default, fix bugs deleting RRDP repo files.

+Remove unnecessary functions at 'visited_uris.h', rename function that deletes local files.
+Refactor the way the old repository files related to an RRDP URI are deleted, instead of deleting the 'best guess' of the root dir, delete each root dir of the mft uris stored at visited uris struct. The daemon will do its best effort to remove the files.
+Update year 2019 refs by 2020.
+Use 'root-except-ta' rsync strategy as default (and update docs as well), to prevent rsyncs to overwrite repositories fetched via RRDP.
+Remove 'create_snapshot' logic from 'rrdp_parser', wasn't of too much help since the 'If-Modified-Since' impl already avoids to load unnecessary data.
+Remove local repository files related to an RRDP URI only on session ID updates; also, reset RSYNC visited URIs of a TAL if an RRDP repository sync fails, this helps to refresh the repo via rsync (if rsync is the secondary option to fetch it).
+Fix 'tal_test.c' error comparing loaded URIs.