Otherwise we'd end up trying to delete the wrong connection if a
connection made before us is deleted.
Fixes _some_ RequestServer spins (though not all...).
This commit also adds a small debug mechanism to RequestServer (which
can be enabled by turning REQUEST_SERVER_DEBUG on), that can dump all
the current active connections in the cache, what they're doing, and how
long they've been doing that by sending it a SIGINFO.
This makes connections (particularly TLS-based ones) do the handshaking
stuff only once.
Currently the cache is configured to keep at most two connections evenly
balanced in queue size, and with a grace period of 10s after the last
queued job has finished (after which the connection will be dropped).
