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.