Add support for determining if new attempt in existing transaction

Currently, gocbcore and query do not know if they are initiating a new attempt in a new transaction, or a new attempt in an existing transaction.  This is by design - until now there has been no need to separate the two.

For MB-50167, gocbcore needs to know if it's a new attempt in an existing transaction, so let's add this to the txdata sent at BEGIN WORK.  This will allow accurate stats tracking regarding new attempts vs new transactions.

I guess query needs to add a separate N1QL parameter so that users trying to run transactions purely with N1QL statements can still send this information and get correct stats.  Pinging Kamini Jagtiani for awareness.