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/main.h

@@ -245,14 +245,7 @@ struct CDiskTxPos : public CDiskBlockPos
 };
 
 
-
-enum GetMinFee_mode
-{
-    GMF_RELAY,
-    GMF_SEND,
-};
-
-int64_t GetMinFee(const CTransaction& tx, unsigned int nBytes, bool fAllowFree, enum GetMinFee_mode mode);
+int64_t GetMinRelayFee(const CTransaction& tx, unsigned int nBytes, bool fAllowFree);
 
 //
 // Check transaction inputs, and make sure any