ENG-4161: Part 1.  Add isReadOnly() to the InitiateResponseMessage so
that we'll be able to determine whether or not the response was for a
short-circuit read or not.
ENG-4161: Part 2.  SpScheduler should not attempt to look up a
duplicate counter for read-only transactions.  We could end up in this
weird state during repair where a newly promoted master would have
both a short-circuit read and a write with the same sphandle, and the
read would incorrectly get sent to the duplicate counter and result in
a hash mismatch.
ENG-4161: Don't log SP read-only transactions to the repair log.