kolab-util-kconv: changed checksumming algo to be used for Kolab_conv_mail
kolab-util-kconv: changed checksumming algo to be used for Kolab_conv_mail

* in kolab_util_kconv_kconvmail_checksum(), changed checksumming algo
  to be used for Kolab_conv_mail change detection from SHA256 to
  SHA1
* added explanatory comment (no need for strong hashes since we're not
  doing crypto here but just change detection)

GNOME/evolution-kolab,GNOME/evolution-kolab