Problem: should be possible to clear cached file digest on file instances

Solution: call zdigest_destroy on zfile_restat
and inline zfile_close in zfile_digest

Other solutions are possible:
* provide explicit method to clear the file digest
* don't cache the digest
Merge pull request #858 from skaes/zfile_restat_should_reset_file_digest

Problem: should be possible to clear cached file digest on file instances