Allow block announcements with headers

This replaces using inv messages to announce new blocks, when a peer requests
(via the new "sendheaders" message) that blocks be announced with headers
instead of inv's.

Since headers-first was introduced, peers send getheaders messages in response
to an inv, which requires generating a block locator that is large compared to
the size of the header being requested, and requires an extra round-trip before
a reorg can be relayed.  Save time by tracking headers that a peer is likely to
know about, and send a headers chain that would connect to a peer's known
headers, unless the chain would be too big, in which case we revert to sending
an inv instead.

Based off of @sipa's commit to announce all blocks in a reorg via inv,
which has been squashed into this commit.

Rebased-by: Pieter Wuille

src/chain.cpp

@@ -51,6 +51,9 @@ CBlockLocator CChain::GetLocator(const CBlockIndex *pindex) const {
 }
 
 const CBlockIndex *CChain::FindFork(const CBlockIndex *pindex) const {
+    if (pindex == NULL) {
+        return NULL;
+    }
     if (pindex->nHeight > Height())
         pindex = pindex->GetAncestor(Height());
     while (pindex && !Contains(pindex))