Separate selection of checksum_type for insync and validation

In PUP-1208, chuck mentions that it's not really appropriate to select `mtime/ctime` for validating whether the file stream received matches the file on disk, it mainly makes sense as a check for whether to update a file target from the source on server. If you want to use a fast check like `mtime` for deciding whether to update, it should still be possible to select something else for checking whether to update a file.

He proposes adding a new `update_checksum` property to separate specifying those checksums.