Direct headers announcement (rebase of #6494) #7129

1. 
   sipa commented at 11:10 am on November 29, 2015: member

   This is a rebase of #6494, with one line (which @sdaftuar commented he forgot to remove) deleted.

   It also adds a commit to add the change to the bips document and the release notes.

2. 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

   50262d8953
3. Documentation updates for BIP 130 49fb8e89b7
4. 
   gmaxwell commented at 11:20 am on November 29, 2015: contributor

   DOC ACK.
5. sipa merged this on Nov 29, 2015
6. sipa closed this on Nov 29, 2015

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7. sipa referenced this in commit 5d5ef3a4cf on Nov 29, 2015
8. snogcel referenced this in commit a8a21986db on Aug 11, 2016
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sipa gmaxwell