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/net.h

@@ -390,6 +390,9 @@ public:
     std::vector<CInv> vInventoryToSend;
     CCriticalSection cs_inventory;
     std::multimap<int64_t, CInv> mapAskFor;
+    // Used for headers announcements - unfiltered blocks to relay
+    // Also protected by cs_inventory
+    std::vector<uint256> vBlockHashesToAnnounce;
 
     // Ping time measurement:
     // The pong reply we're expecting, or 0 if no pong expected.
@@ -504,6 +507,12 @@ public:
         }
     }
 
+    void PushBlockHash(const uint256 &hash)
+    {
+        LOCK(cs_inventory);
+        vBlockHashesToAnnounce.push_back(hash);
+    }
+
     void AskFor(const CInv& inv);
 
     // TODO: Document the postcondition of this function.  Is cs_vSend locked?