Display effective IO backend in 'cbc-version'

Change-Id: Id3ba6ad2a529eedac89b8c2330e63a0a1a04149f
Reviewed-on: http://review.couchbase.org/28488
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>