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.