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/rpc-tests/test_framework/mininode.py

@@ -751,8 +751,8 @@ class msg_inv(object):
 class msg_getdata(object):
     command = "getdata"
 
-    def __init__(self):
-        self.inv = []
+    def __init__(self, inv=None):
+        self.inv = inv if inv != None else []
 
     def deserialize(self, f):
         self.inv = deser_vector(f, CInv)
@@ -905,6 +905,20 @@ class msg_mempool(object):
     def __repr__(self):
         return "msg_mempool()"
 
+class msg_sendheaders(object):
+    command = "sendheaders"
+
+    def __init__(self):
+        pass
+
+    def deserialize(self, f):
+        pass
+
+    def serialize(self):
+        return ""
+
+    def __repr__(self):
+        return "msg_sendheaders()"
 
 # getheaders message has
 # number of entries
@@ -990,6 +1004,17 @@ class NodeConnCB(object):
     def __init__(self):
         self.verack_received = False
 
+    # Spin until verack message is received from the node.
+    # Tests may want to use this as a signal that the test can begin.
+    # This can be called from the testing thread, so it needs to acquire the
+    # global lock.
+    def wait_for_verack(self):
+        while True:
+            with mininode_lock:
+                if self.verack_received:
+                    return
+            time.sleep(0.05)
+
     # Derived classes should call this function once to set the message map
     # which associates the derived classes' functions to incoming messages
     def create_callback_map(self):
@@ -1084,7 +1109,7 @@ class NodeConn(asyncore.dispatcher):
         "regtest": "\xfa\xbf\xb5\xda"    # regtest
     }
 
-    def __init__(self, dstaddr, dstport, rpc, callback, net="regtest"):
+    def __init__(self, dstaddr, dstport, rpc, callback, net="regtest", services=1):
         asyncore.dispatcher.__init__(self, map=mininode_socket_map)
         self.log = logging.getLogger("NodeConn(%s:%d)" % (dstaddr, dstport))
         self.dstaddr = dstaddr
@@ -1102,6 +1127,7 @@ class NodeConn(asyncore.dispatcher):
 
         # stuff version msg into sendbuf
         vt = msg_version()
+        vt.nServices = services
         vt.addrTo.ip = self.dstaddr
         vt.addrTo.port = self.dstport
         vt.addrFrom.ip = "0.0.0.0"