Test Details Overview

Keep the handshake cookie in `tcp_server` instead of a shared table (#5278)

A handshake cookie belongs to one connection, so the server of that
connection now holds it: `handshake_cookie` is set when the connection
sends its query and taken when a response arrives. A connection sends
one query, the side that opened it in its request and the side that
accepted it next to its response.

`nano::syn_cookies`, which kept the cookies per endpoint next to the
connections and counted them per IP, is removed together with its purge
in the cleanup loop, `syn_cookie_cutoff`, its container info and the
unused `syn_cookies` stat and log types. `prepare_handshake_query ()`
no longer takes an endpoint and always returns a query, and
`verify_handshake_response ()` takes the cookie of the connection the
response arrived on.

A cookie now lasts as long as its connection's handshake, so the
handshake timeout bounds it. A new connection to an endpoint asks with
a cookie of its own whatever became of an earlier connection there,
and the listener's per IP limit is the one bound on handshakes per IP,
which also makes `disable_max_peers_per_ip` apply to them.

The `tcp_server` tests cover a response on a connection that sent no
query, two connections from one IP getting separate cookies, and a
second connection to an endpoint whose first one ended mid handshake.

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Type: commit

Hash: 5bdac0ca50ed25b60594bdd664c5d3e9d6a4bfbe

Created: today

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gr0v1ty/nano-node:5bdac0ca50ed25b60594bdd664c5d3e9d6a4bfbe