Raise default max_clients to 16384, add a recv_buffer option

[williamthome]

Description

Stacked on #196.

max_clients defaulted to 150, which refuses connections long before the VM or the OS is under real pressure. The default moves to 16384. Saturation memory scales as max_clients × recv_buffer, so the socket receive buffer that was hardcoded at 64 KB becomes the recv_buffer listener option, giving a direct lever to trade body-path throughput for per-connection memory rather than forcing the cap back down.

roadrunner:start_listener(api, #{port => 8080, max_clients => 65536, recv_buffer => 16384}, Routes).

The higher default puts the binding limit on the OS rather than the listener: the effective cap is min(max_clients, ulimit -n), and a host left at a 1024 descriptor limit will now reach emfile before reaching max_clients. That is documented on the option, and it is observable rather than silent, since the acceptors emit [roadrunner, listener, accept_error] at the descriptor ceiling and keep accepting (#196). Deployments that want the old behaviour of a clean refusal at a fixed low ceiling can set max_clients explicitly.

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