use POSIX::_exit instead of exit in forked child

use POSIX::_exit instead of exit in forked child

This avoids calling the END blocks and destructor routines most of
which have nothing to do with the fork, so it cannot be assumed to be
safe to do from the fork.

Move the call to pve_rados_shutdown from the destructor routine,
which will not be called by the child anymore. Make sure the cleanup
is also done when the worker dies.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>