Increase compat with older versions

- Forward-port 2.3.1 compat ABI
  This is to be in sync with the latest libcouchbase master with the 'v2'
  IOPS plugin. See CCBC-392

- Restore v0 getreplica fields to their older names
  No code should be using this structure still, but this does help restore
  API compat

- Re-add lcb_flush_buffers() as a noop
  This function has not really done anything since version 2.1

Change-Id: Ib43a29f07af0244d0e5976e0e99886f079386bc7
Reviewed-on: http://review.couchbase.org/38209
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>