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.

doc/release-notes.md

@@ -1,6 +1,26 @@
 (note: this is a temporary file, to be added-to by anybody, and moved to
 release-notes at release time)
 
+Transaction fee changes
+=======================
+
+This release automatically estimates how high a transaction fee (or how
+high a priority) transactions require to be confirmed quickly. The default
+settings will create transactions that confirm quickly; see the new
+'txconfirmtarget' setting to control the tradeoff between fees and
+confirmation times.
+
+Prior releases used hard-coded fees (and priorities), and would
+sometimes create transactions that took a very long time to confirm.
+
+
+New Command Line Options
+========================
+
+-txconfirmtarget=n : create transactions that have enough fees (or priority)
+so they are likely to confirm within n blocks (default: 1). This setting
+is over-ridden by the -paytxfee option.
+
 New RPC methods
 ===============