CLEANUP: cli: simplify the request parser a little bit

CLEANUP: cli: simplify the request parser a little bit

stats_sock_parse_request() was renamed cli_parse_request(). It now takes
an appctx instead of a stream interface, and presets ->st2 to 0 so that
most handlers will not have to set it anymore. The io_handler is set by
default to the keyword's IO handler so that the parser can simply change
it without having to rewrite the new state.