Use fee/priority estimates in wallet CreateTransaction

The wallet now uses the mempool fee estimator with a new
command-line option: -txconfirmtarget (default: 1) instead
of using hard-coded fees or priorities.

A new bitcoind that hasn't seen enough transactions to estimate
will fall back to the old hard-coded minimum priority or
transaction fee.

-paytxfee option overrides -txconfirmtarget.

Relaying and mining code isn't changed.

For Qt, the coin control dialog now uses priority estimates to
label transaction priority (instead of hard-coded constants);
unspent outputs were consistently labeled with a much higher
priority than is justified by the free transactions actually
being accepted into blocks.

I did not implement any GUI for setting -txconfirmtarget; I would
suggest getting rid of the "Pay transaction fee" GUI and replace
it with either "target number of confirmations" or maybe
a "faster confirmation <--> lower fee" slider or select box.

src/wallet.h

@@ -25,12 +25,15 @@
 
 // Settings
 extern CFeeRate payTxFee;
+extern unsigned int nTxConfirmTarget;
 extern bool bSpendZeroConfChange;
 
 // -paytxfee default
 static const int64_t DEFAULT_TRANSACTION_FEE = 0;
 // -paytxfee will warn if called with a higher fee than this amount (in satoshis) per KB
 static const int nHighTransactionFeeWarning = 0.01 * COIN;
+// Largest (in bytes) free transaction we're willing to create
+static const unsigned int MAX_FREE_TRANSACTION_CREATE_SIZE = 1000;
 
 class CAccountingEntry;
 class CCoinControl;
@@ -265,6 +268,8 @@ public:
     std::string SendMoney(CScript scriptPubKey, int64_t nValue, CWalletTx& wtxNew);
     std::string SendMoneyToDestination(const CTxDestination &address, int64_t nValue, CWalletTx& wtxNew);
 
+    static int64_t GetMinimumFee(unsigned int nTxBytes, unsigned int nConfirmTarget, const CTxMemPool& pool);
+
     bool NewKeyPool();
     bool TopUpKeyPool(unsigned int kpSize = 0);
     void ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& keypool);