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/qt/rpcconsole.cpp

@@ -611,7 +611,6 @@ void RPCConsole::updateNodeDetail(const CNodeCombinedStats *stats)
     ui->peerSubversion->setText(QString::fromStdString(stats->nodeStats.cleanSubVer));
     ui->peerDirection->setText(stats->nodeStats.fInbound ? tr("Inbound") : tr("Outbound"));
     ui->peerHeight->setText(QString("%1").arg(stats->nodeStats.nStartingHeight));
-    ui->peerSyncNode->setText(stats->nodeStats.fSyncNode ? tr("Yes") : tr("No"));
 
     // This check fails for example if the lock was busy and
     // nodeStateStats couldn't be fetched.