make "internal" model properties enumerable

Internal non-enumerable properties of game objects (such as "id",
"slot" for items etc) need to be configurable, otherwise they break
Object.keys on objref proxies. See the inline comments for more
details, and this somewhat related issue:
https://github.com/tvcutsem/harmony-reflect/issues/25
catch errors in async persistence operations

When processing write/del persistence operations at the end of a
request, async.each takes them out of the fiber context, and they are
not covered by the session domain either. The persistence ops
themselves report errors through their callback, which is already
handled, but the surrounding steps (specifically: serialize) may throw
errors, so catch them.
This should probably be handled in a cleaner, more elegant way.