Use exceptfds in multithread plugin

Change-Id: I7104621be2cc49245868c0c00d1f19f1b7b1e775
Reviewed-on: http://review.couchbase.org/31681
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>