Optimize JSON-RPC getblockhash

- If the height is in the first half, start at the genesis block and go up, rather than at the top
- Cache the last lookup and use it as a reference point if it's close to the next request, to make linear lookups always fast

src/main.h

@@ -88,6 +88,7 @@ FILE* OpenBlockFile(unsigned int nFile, unsigned int nBlockPos, const char* pszM
 FILE* AppendBlockFile(unsigned int& nFileRet);
 bool LoadBlockIndex(bool fAllowNew=true);
 void PrintBlockTree();
+CBlockIndex* FindBlockByHeight(int nHeight);
 bool ProcessMessages(CNode* pfrom);
 bool SendMessages(CNode* pto, bool fSendTrickle);
 bool LoadExternalBlockFile(FILE* fileIn);