Implement "feefilter" P2P message.

The "feefilter" p2p message is used to inform other nodes of your mempool min fee which is the feerate that any new transaction must meet to be accepted to your mempool.  This will allow them to filter invs to you according to this feerate.

src/txmempool.cpp

@@ -771,6 +771,16 @@ bool CTxMemPool::lookup(uint256 hash, CTransaction& result) const
     return true;
 }
 
+bool CTxMemPool::lookupFeeRate(const uint256& hash, CFeeRate& feeRate) const
+{
+    LOCK(cs);
+    indexed_transaction_set::const_iterator i = mapTx.find(hash);
+    if (i == mapTx.end())
+        return false;
+    feeRate = CFeeRate(i->GetFee(), i->GetTxSize());
+    return true;
+}
+
 CFeeRate CTxMemPool::estimateFee(int nBlocks) const
 {
     LOCK(cs);