Allow to choose plugin constructor without using dlopen(3)

Change-Id: Ic024904a161faf2708e11286c5759eec3aee00fe
Reviewed-on: http://review.couchbase.org/26115
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>