MB-11286: added OS pid of child to messages from child_erlang

There's hope that MB-11286 might be better diagnosed by doing it.

Change-Id: I8f103b3d688e929c16e7659d1d90c1e50173f7d3
Reviewed-on: http://review.couchbase.org/37987
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>