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/rpcnet.cpp

@@ -139,7 +139,6 @@ Value getpeerinfo(const Array& params, bool fHelp)
             obj.push_back(Pair("banscore", statestats.nMisbehavior));
             obj.push_back(Pair("syncheight", statestats.nSyncHeight));
         }
-        obj.push_back(Pair("syncnode", stats.fSyncNode));
         obj.push_back(Pair("whitelisted", stats.fWhitelisted));
 
         ret.push_back(obj);