This commit reworks the getheaders handling a bit to clean it up and match
the reference implementation handling. In particular, in addition to the
normal handling where headers starting after the block locator up to the
stop hash are served, when no locator hashes are provided, the stop hash
acts as a way to specifically request that header. Next, an empty headers
message is sent when no hashes provided by the block locator can be
found. Finally, there was a bug that was limiting the number of headers
that could requested at once to 500 instead of the expected 2000.
Rather than only setting the services field for inbound peers, set it for
all peers. This field referes to the remote peer's services regardless of
inbound or outbound.
This commit significantly reworks the fetching code to interop better with
bitcoind. In particular, when an inventory message is sent, and the
remote peer requests the final block, the remote peer sends the current
end of the main chain to signal that there are more blocks to get.
Previously this code was automatically requesting more blocks when the
number of in-flight blocks was under a certain threshold. The original
approach does help alleviate delays in the "request final, wait for
orphan, request more" round trip, but due to the aforementioned mechanism,
it leads to double requests and other subtle issues.
This commit modifies the input message handler so that when a remote peer
sends a block, no further messages from that peer are accepted until the
block has been fully processed and therefore known good or bad. This
helps prevent a malicious peer from queueing up a bunch of bad blocks
before disconnecting (or being disconnected) and wasting memory.
Additionally, this behavior is depended on by at least the block
acceptance test tool as the reference implementation processes blocks in
the same thread and therefore blocks further messages until the block has
been fully processed as well.
