RESTEnvironment: better SIGCHLD handling in AnyEvent event loop

RESTEnvironment: better SIGCHLD handling in AnyEvent event loop

when we're in an API server that uses AnyEvent, we must postpone
the worker_reaper, since it calls 'active_workers' which might already
be called and then we're inside the lock twice (flocks are per process
for us, see PVE::Tools::lock_file)

This resulted in an error like this:
close (rename) atomic file '/var/log/pve/tasks/active' failed: No such file or directory

We use the fact that only 'pub' and 'priv' RESTEnvironment types are an
api server with anyevent. For other types we call it like before.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>