Restore ABI compatibility for 'subclassing' I/O plugins

This necessarily introduces a set of hacks to deal with, but is
generally safe over the option of breaking some older builds.

Change-Id: I9bc00436470573d44287da6c74758798fa134bd8
Reviewed-on: http://review.couchbase.org/43628
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>