Fixup file modification timestamps to optimize failover from RRDP to RSYNC

Fixup file modification timestamps to optimize failover from RRDP to RSYNC

In the RSYNC protocol a file's last modification time and its size are
used to determine whether sending a (partial) copy over the wire is needed.
Previously, when RRDP data structures are serialized to disk, the mtime of
files in DIR_VALID ended up being UTIME_NOW.

Thus, the mtimes of files obtained through RRDP will never match the mtimes
of the same files available through RSYNC - causing each and every file to
be added to the file transfer list.

Instead, use the internal timestamps of RPKI files as the last modified
timestamp. Specifically, for Signed Objects (ROAs, MFTs, GBRs, TAKs, ASPAs)
the CMS signing-time, for .cer files the X.509 notBefore, and for .crl files
the CRL lastUpdate. This results in a surprising optimization for the number
files which have to be transfered.

OK claudio@