fix: also sync nested pipeline when a transaction is active

fix: also sync nested pipeline when a transaction is active

Upon enter, transaction() checks the in-transaction status to determine
if a BEGIN (when IDLE) or a SAVEPOINT (otherwise) should be inserted.
However, in pipeline mode, if the pipeline is in "implicit transaction"
(i.e. it has no active BEGIN) and statements have been sent, the
in-transaction status might be ACTIVE if those statements have not yet
completed (typically, when results have not been fetched).

By issuing a sync() before entering a nested pipeline (which will happen
when entering transaction(), if already within a pipeline), we force
completion of those statements, and thus get a predictable
in-transaction status before entering the transaction() block.

Closes #374.