Prioritize existing clients over accepting new

[m-ronnblom]

The Pathfinder server should prioritize processing messages on existing clients over accepting and finishing the handshake with new clients.

With Pathfinder protocol v3 and its Pathfinder protocol-level liveness checking, attending to existing clients becomes more important, since otherwise the clients will terminate the connection and reconnect.

In a scenario where a large number clients attempt to connect at roughly the same time, expensive hand shake operations (e.g., TLS hello processing) may starve server or client-initiated track query processing, causing existing client connections to be dropped (in turn adding to the handshake load).