Behaviour for nhttp's proc_lib protocol loop modules.

A protocol module owns one phase of a connection process's lifetime: nhttp_conn (pre-classification socket wait), nhttp_conn_h1, nhttp_conn_h2, nhttp_conn_h3, and nhttp_conn_ws_h1 (post-upgrade WebSocket session). The connection process never changes pid across phases; control is handed from one module's loop to the next by a tail call into loop/3.

loop/3 is the (re-)entry point of the module's receive loop: nhttp_conn tail-calls it after protocol classification, and proc_lib:hibernate/3 wake-ups re-enter through it or a module wake function that converges on it. The state term is owned by the module that is currently looping; callers treat it as opaque.

The three system_* callbacks are the sys contract for a special process (see sys:handle_system_msg/6, called with the looping module as the callback module): system_continue/3 must re-enter loop/3, system_terminate/4 must run the connection's termination path and exit, and system_code_change/4 returns the (unchanged) state via the ?NHTTP_SYSTEM_CODE_CHANGE_NOOP macro from nhttp_proc_lib.hrl.