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
Instead of only checking height to decide whether to disable script checks,
actually check whether a block is an ancestor of a checkpoint, up to which
headers have been validated. This means that we don't have to prevent
accepting a side branch anymore - it will be safe, just less fast to
do.
We still need to prevent being fed a multitude of low-difficulty headers
filling up our memory. The mechanism for that is unchanged for now: once
a checkpoint is reached with headers, no headers chain branching off before
that point are allowed anymore.
