Headers-first synchronization

Here's a first (well, second) version of a fully-functional 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.
* Some new fields in getpeerinfo:
  * 'syncheight': the height up to which we've validated this peer's headers.
  * 'commonheight': the height up to which we have all blocks in common with this peer.
  * 'inflight': the heights we're currently requesting from this peer.
* 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 and comments.
* Reindexing support for out-of-order blocks on disk.

Yes, that's completely expected. The point is that other peers managed to progress past the whole window, while we're still waiting on this one peer. There is nothing to be gained from keeping it connected, as we're effectively faster by not downloading from them at all.

You could argue that we should wait for a short while to give them a chance to finish the block we're currently fetching, but even in that case, it means wasting time not using bandwidth that other peer could be providing us.