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/qt/coincontroldialog.h

@@ -19,6 +19,7 @@ namespace Ui {
 }
 class WalletModel;
 class CCoinControl;
+class CTxMemPool;
 
 class CoinControlDialog : public QDialog
 {
@@ -32,7 +33,7 @@ public:
 
     // static because also called from sendcoinsdialog
     static void updateLabels(WalletModel*, QDialog*);
-    static QString getPriorityLabel(double);
+    static QString getPriorityLabel(const CTxMemPool& pool, double);
 
     static QList<qint64> payAmounts;
     static CCoinControl *coinControl;