Add a warning if starting a transaction within a transaction

In SDK 2.x, starting a transaction within a running transaction is undefined behavior. In SDK 3.x, there is no real user-facing `start_transaction` anymore, which might mean that spans that used to be transactions in 2.x will become child spans in 3.x.

We should issue a warning if we detect someone wants to `start_transaction` while another transaction is running in 2.x, to give people heads up before upgrading to 3.x.