shared/archive: Wraps cancelFunc to wait until unpacker process has finished in CompressedTarReader
shared/archive: Wraps cancelFunc to wait until unpacker process has finished in CompressedTarReader

Avoids race conditions where context's cancelFunc returns before unpacker has finished.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

Skarlso/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,mjeanson/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,Skarlso/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,mjeanson/lxd,mjeanson/lxd,stgraber/lxd,hallyn/lxd,Skarlso/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,atxwebs/lxd,lxc/lxd,Skarlso/lxd,atxwebs/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,atxwebs/lxd