This new method will serve as a single point of access to the mempool reactor for incoming transactions.

Currently to add a transaction we call CListMempool.CheckTx(...) directly, either from the broadcast_tx_* RPC endpoints or from the Receive(...) method in the reactor. The goal of TryAddTx is to have a single method in the reactor where in the future we can put common logic for adding transactions, such as handling errors or sending new messages. In particular, this refactoring will be needed by the implementations of mempool lanes and the DOG protocol.