Add a new flag for API check instead of clearing prepared flag which may incorrectly force a roll back

Currently, the WT_TXN_PREPARED flag is temporarily cleared in the function __txn_search_prepared_op. If eviction is currently happening and experiences cache pressure, it may try to rollback an active transaction. While the flag is temporarily cleared, the __wt_txn_is_blocking function will not recognise that the current transaction is a prepared transaction, and triggers a rollback with the "oldest pinned transaction ID rolled back for eviction" reason.