fix #3359: fix blocking writes in async code during pxar create

fix #3359: fix blocking writes in async code during pxar create

in commit `asyncify pxar create_archive`, we changed from a
separate thread for creating a pxar to using async code, but the
StdChannelWriter used for both pxar and catalog can block, which
may block the tokio runtime for single (and probably dual) core
environments

this patch adds a wrapper struct for any writer that implements
'std::io::Write' and wraps the write calls with 'block_in_place'
so that if called in a tokio runtime, it knows that this code
potentially blocks

Fixes: 6afb60abf557 ("asyncify pxar create_archive")
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>