Database micro-optimization for "tx" network message

Open database once per "tx" message, rather than multiple times,
in the case of orphan transaction presence.

As a side effect, a now-unused CTransaction::AcceptToMemoryPool()
variant is removed.

src/main.h

@@ -684,7 +684,6 @@ public:
     bool ClientConnectInputs();
     bool CheckTransaction() const;
     bool AcceptToMemoryPool(CTxDB& txdb, bool fCheckInputs=true, bool* pfMissingInputs=NULL);
-    bool AcceptToMemoryPool(bool fCheckInputs=true, bool* pfMissingInputs=NULL);
 
 protected:
     const CTxOut& GetOutputFor(const CTxIn& input, const MapPrevTx& inputs) const;