backup: implement compression of archived files.

New "backend" parameter to zip archived files: "compress".
Compressed file names ends with 'z' after their fid.
Code quality: replace every hardcoded check of sscanf(RFID) != 3 with sscanf(RFID) != FID_SCAN_CNT.
EntryProc EFIX: pushing pipeline operation from the pipeline itself may result in deadlock.

Triggering a parent update operation when the pipeline is full (when max_pending_operation is reached)
results in blocking the current operation, so causing a deadlock.

Implemented solution: no longer trigger parent update when a parent is not known,
which is not supposed to happen. However, deal with the normal case of an
obsolete parent_id from the changelog that no longer exists. In this case,
we just update the parent/name information for the current entry.