properly restart compression when encrypting

properly restart compression when encrypting

Separate finalization of a zlib block from creation of a new one.
Otherwise, we end up with trailing data from the last object that
lingers in the write buffer and gets flushed to the archive after
the next encrypted object has been initialized.

Also get rid of the “new_compression_block” wrapper which
needlessly complicated things.

Special precautions must be taken for the PAX format. Due to its
requirement of a global archive header, TarFile will write to the
stream prior to initialization that is performed in addfile().
Thus, initialize compression before the PAX header is being
written and properly restart compression for the first object
committed to the archive or volume.