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

qa/pull-tester/rpc-tests.py

@@ -91,6 +91,7 @@ testScripts = [
     'p2p-fullblocktest.py',
     'blockchain.py',
     'disablewallet.py',
+    'sendheaders.py',
 ]
 testScriptsExt = [
     'bip65-cltv.py',