Add BroadcastTransaction utility usage in Chain interface

Access through a broadcastTransaction method.
Add a wait_callback flag to turn off race protection when wallet
already track its tx being in mempool

Standardise highfee, absurdfee variable name to max_tx_fee

We drop the P2P check in BroadcastTransaction as g_connman is only
called by RPCs and the wallet scheduler, both of which are initialized
after g_connman is assigned and stopped before g_connman is reset.

src/interfaces/chain.h

@@ -167,6 +167,11 @@ public:
     //! Relay transaction.
     virtual void relayTransaction(const uint256& txid) = 0;
 
+    //! Transaction is added to memory pool, if the transaction fee is below the
+    //! amount specified by max_tx_fee, and broadcast to all peers if relay is set to true.
+    //! Return false if the transaction could not be added due to the fee or for another reason.
+    virtual bool broadcastTransaction(const CTransactionRef& tx, std::string& err_string, const CAmount& max_tx_fee, bool relay) = 0;
+
     //! Calculate mempool ancestor and descendant counts for the given transaction.
     virtual void getTransactionAncestry(const uint256& txid, size_t& ancestors, size_t& descendants) = 0;