[5.0] fix crash/UB when exceeding http-max-requests-in-flight

[spoonincode]

When verify_max_requests_in_flight() indicates we've exceeded max requests in flight, send_busy_response(), via send_response(), queues up an async_write() on the socket. But once this async_write() returns, do_eof() would be called which closes the socket while operations are pending. This causes bad things to happen.

Instead, set the keep_alive(false) on the response, which flags needs_eof() so that,

leap/plugins/http_plugin/include/eosio/http_plugin/beast_http_session.hpp

Line 478 in 3394e0b

bool close = !(plugin_state_->keep_alive) || res_->need_eof();

will be set to true, which means the socket will be properly disposed of once the async_write() completes,

leap/plugins/http_plugin/include/eosio/http_plugin/beast_http_session.hpp

Lines 368 to 372 in 3394e0b

	if(close) {
	// This means we should close the connection, usually because
	// the response indicated the "Connection: close" semantic.
	return do_eof();
	}

[ericpassmore]

Note:start
group: STABILITY
category: BUG
summary: Guard against errors when exceeding http-max-requests-in-flight
Note:end