avoid spamming logs by xdcr_upr_streamer on crashes in better way

proc_lib is printing neighbors mailboxes only for neighbors that don't
trap exits. So now instead of setting xdcr_upr_streamer processes to
be sensitive to avoid spamming logs by their mailbox contents, we now
simply trap exit in direct child of xdc_vbucket_rep. That avoids
spamming without impeding diagnostics.

Change-Id: Ia08a99aadf41957b4370f4d4d04ffac9598a69a6
Reviewed-on: http://review.couchbase.org/35531
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>