estimatefee / estimatepriority RPC methods

New RPC methods: return an estimate of the fee (or priority) a
transaction needs to be likely to confirm in a given number of
blocks.

Mike Hearn created the first version of this method for estimating fees.
It works as follows:

For transactions that took 1 to N (I picked N=25) blocks to confirm,
keep N buckets with at most 100 entries in each recording the
fees-per-kilobyte paid by those transactions.

(separate buckets are kept for transactions that confirmed because
they are high-priority)

The buckets are filled as blocks are found, and are saved/restored
in a new fee_estiamtes.dat file in the data directory.

A few variations on Mike's initial scheme:

To estimate the fee needed for a transaction to confirm in X buckets,
all of the samples in all of the buckets are used and a median of
all of the data is used to make the estimate. For example, imagine
25 buckets each containing the full 100 entries. Those 2,500 samples
are sorted, and the estimate of the fee needed to confirm in the very
next block is the 50'th-highest-fee-entry in that sorted list; the
estimate of the fee needed to confirm in the next two blocks is the
150'th-highest-fee-entry, etc.

That algorithm has the nice property that estimates of how much fee
you need to pay to get confirmed in block N will always be greater
than or equal to the estimate for block N+1. It would clearly be wrong
to say "pay 11 uBTC and you'll get confirmed in 3 blocks, but pay
12 uBTC and it will take LONGER".

A single block will not contribute more than 10 entries to any one
bucket, so a single miner and a large block cannot overwhelm
the estimates.

src/txmempool.h

@@ -11,6 +11,13 @@
 #include "core.h"
 #include "sync.h"
 
+inline bool AllowFree(double dPriority)
+{
+    // Large (in bytes) low-priority (new, small-coin) transactions
+    // need a fee.
+    return dPriority > COIN * 144 / 250;
+}
+
 /** Fake height value used in CCoins to signify they are only in the memory pool (since 0.8) */
 static const unsigned int MEMPOOL_HEIGHT = 0x7FFFFFFF;
 
@@ -41,6 +48,8 @@ public:
     unsigned int GetHeight() const { return nHeight; }
 };
 
+class CMinerPolicyEstimator;
+
 /*
  * CTxMemPool stores valid-according-to-the-current-best-chain
  * transactions that may be included in the next block.
@@ -56,6 +65,7 @@ class CTxMemPool
 private:
     bool fSanityCheck; // Normally false, true if -checkmempool or -regtest
     unsigned int nTransactionsUpdated;
+    CMinerPolicyEstimator* minerPolicyEstimator;
 
 public:
     mutable CCriticalSection cs;
@@ -63,6 +73,7 @@ public:
     std::map<COutPoint, CInPoint> mapNextTx;
 
     CTxMemPool();
+    ~CTxMemPool();
 
     /*
      * If sanity-checking is turned on, check makes sure the pool is
@@ -76,6 +87,8 @@ public:
     bool addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry);
     void remove(const CTransaction &tx, std::list<CTransaction>& removed, bool fRecursive = false);
     void removeConflicts(const CTransaction &tx, std::list<CTransaction>& removed);
+    void removeForBlock(const std::vector<CTransaction>& vtx, unsigned int nBlockHeight,
+                        std::list<CTransaction>& conflicts);
     void clear();
     void queryHashes(std::vector<uint256>& vtxid);
     void pruneSpent(const uint256& hash, CCoins &coins);
@@ -95,6 +108,16 @@ public:
     }
 
     bool lookup(uint256 hash, CTransaction& result) const;
+
+    // Estimate fee rate needed to get into the next
+    // nBlocks
+    CFeeRate estimateFee(int nBlocks) const;
+    // Estimate priority needed to get into the next
+    // nBlocks
+    double estimatePriority(int nBlocks) const;
+    // Write/Read estimates to disk
+    bool WriteFeeEstimates(CAutoFile& fileout) const;
+    bool ReadFeeEstimates(CAutoFile& filein);
 };
 
 /** CCoinsView that brings transactions from a memorypool into view.