Making the uniq_id test helper return truly unique ids. Updated method taken from client source.

Change-Id: I66924b043400b2e6863b154fe8e0363f2b101e6f
Reviewed-on: http://review.couchbase.org/30991
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>