Headers-first synchronization

Many changes:
* Do not use 'getblocks', but 'getheaders', and use it to build a headers tree.
* Blocks are fetched in parallel from all available outbound peers, using a
  limited moving window. When one peer stalls the movement of the window, it is
  disconnected.
* No more orphan blocks. At all. We only ever request a block for which we have
  verified the headers, and store it to disk immediately. This means that a
  disk-fill attack would require PoW.
* Require protocol version 31800 for every peer (released in december 2010).
* No more syncnode (we sync from everyone we can, though limited to 1 during
  initial *headers* sync).
* Introduce some extra named constants, comments and asserts.

src/net.h

@@ -158,7 +158,6 @@ public:
     int nStartingHeight;
     uint64_t nSendBytes;
     uint64_t nRecvBytes;
-    bool fSyncNode;
     bool fWhitelisted;
     double dPingTime;
     double dPingWait;
@@ -276,10 +275,7 @@ protected:
 
 public:
     uint256 hashContinue;
-    CBlockIndex* pindexLastGetBlocksBegin;
-    uint256 hashLastGetBlocksEnd;
     int nStartingHeight;
-    bool fStartSync;
 
     // flood relay
     std::vector<CAddress> vAddrToSend;