also: remove attic bug support code from borg check.

borg transfer removes the acl key. we do not run borg check on old repos.

hack: see the docstring of ZLIB_legacy class.

New clean ZLIB class that works as every other compressor.

ZLIB ID 0x0500, ZLIB_legacy ID 0x.8..

Item.hlid: same id, same hardlink (xxh64 digest)
Item.hardlink_master: not used for new archives any more
Item.source: not used for hardlink slaves any more

…gbackup#2323

Timestamp scales to 64 or 96bit serialization formats, that should be enough for everybody.

We use this in archived items and also in the files cache.

borg now has the chunks list in every item with content.
due to the symmetric way how borg now deals with hardlinks using
item.hlid, processing gets much simpler.

but some places where borg deals with other "sources" of hardlinks
still need to do some hardlink management:
borg uses the HardLinkManager there now (which is not much more
than a dict, but keeps documentation at one place and avoids some
code duplication we had before).

item.hlid is computed via hardlink_id function.

support hardlinked symlinks, fixes borgbackup#2379
as we use item.hlid now to group hardlinks together,
there is no conflict with the item.source usage for
symlink targets any more.

2nd+ hardlinks now add to the files count as did the 1st one.
for borg, now all hardlinks are created equal.
so any hardlink item with chunks now adds to the "file" count.

ItemFormatter: support {hlid} instead of {source} for hardlinks

now all hardlinked regular file items have chunks.

adapt borg transfer, transferred chunks are set to compression level "unknown".

the inner payload of ObfuscateSize chunks are compressed chunks and need
the same zlib fix and level patching as non-obfuscated compressed chunks.