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.

This almost always (in my testing) results in slowing down the download of the block chain as almost always the stalling node is currently sending the block we are waiting for, and disconnecting it simply means we'll have to discard the part of the block already downloaded and request it from another node which is no more likely to be any faster.