Move fee policy out of core

src/wallet.h

@@ -268,6 +268,7 @@ public:
     std::string SendMoney(CScript scriptPubKey, int64_t nValue, CWalletTx& wtxNew);
     std::string SendMoneyToDestination(const CTxDestination &address, int64_t nValue, CWalletTx& wtxNew);
 
+    static CFeeRate minTxFee;
     static int64_t GetMinimumFee(unsigned int nTxBytes, unsigned int nConfirmTarget, const CTxMemPool& pool);
 
     bool NewKeyPool();