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.cpp

@@ -11,6 +11,7 @@
 #include <net.h>
 #include <net_processing.h>
 #include <node/coin.h>
+#include <node/transaction.h>
 #include <policy/fees.h>
 #include <policy/policy.h>
 #include <policy/rbf.h>
@@ -295,6 +296,14 @@ public:
     {
         RelayTransaction(txid, *g_connman);
     }
+    bool broadcastTransaction(const CTransactionRef& tx, std::string& err_string, const CAmount& max_tx_fee, bool relay) override
+    {
+        const TransactionError err = BroadcastTransaction(tx, err_string, max_tx_fee, relay, /*wait_callback*/ false);
+        // Chain clients only care about failures to accept the tx to the mempool. Disregard non-mempool related failures.
+        // Note: this will need to be updated if BroadcastTransactions() is updated to return other non-mempool failures
+        // that Chain clients do not need to know about.
+        return TransactionError::OK == err;
+    }
     void getTransactionAncestry(const uint256& txid, size_t& ancestors, size_t& descendants) override
     {
         ::mempool.GetTransactionAncestry(txid, ancestors, descendants);