MB-9890: avoid logging states of xdc_vbucket_rep childs

When xdc_vbucket_rep process exits it triggers proc_lib code that
dumps exit reason and other convenience diagnostic. Part of that
diagnostics is dump of neighbor process states that don't trap
exits. And worker sub-processes of xdc_vbucket_rep are such processes
that don't trap exits.

So in order to prevent proc_lib from exposing internals of those
workers, we create them via intermediate processes that do trap exits.

Change-Id: I9a92dacd29fa9655bf840ec4bb225ac6d3b40b45
Reviewed-on: http://review.couchbase.org/42491
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>