+ in __create_symlink: propagate Exception message into logger

Use a symlink to the files with timestamps + length. The symlink doesn't
have timestamps so it doesn't need globbing

It used to be a debug message

The code is now very smart when loading chunks, not need to worry anymore

For some reason we lost those lines in a commit some time ago

Don't trust the chunk number written to file, but instead check each
processed chunk time range. This avoid bugs when resuming work for new
days (in a cotinuous scan that doesn't have a end date)

Previously the overlapping spanned from a chunk to the next one, but this
creates problem when adding new chunks (e.g when extending a scan to new
days)