Preview: headers-first synchronization

Here's a first (well, second) version of a fully-functional headers-first synchronization. It's not ready to be merged (it breaks reindexing, which cannot deal with out-of-order blocks on disk yet).

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.
* Several bugfixes:
  * CChain::GetLocator() didn't work correctly across the CChain end.
  * FindMostWorkChain() failed to remove the actually failed block from setBlockIndexValid.
  * ProcessMessage() failed to act on ban/reject notices resulting from validation failures during the direct execution (CheckBlock, ...).

I'm responding to the comments, not the code— if it's only on duplicates and not just non-requested then I think its safe but only if we release note loudly.  Right now I think eloipool users will end up GBT submitting blocks to multiple Bitcoinds which will then aggressively relay.  It's bandwidth wasteful and it shouldn't be permitted. This is a borderline p2p protocol change, I think.

Though relaynodeclient should do an INV— it's local, this is going to add like... 1ms. And it would also let the client better track when it's the software providing the block.  Same with p2pool. Though indeed, whitelisting also covers these cases.