Issue description

We recently encountered a bug:
when reuseport is enabled by default, during the running of APISIX, modify config.yaml and start without stopping APISIX. This resulted in some workers still using the old config, some workers using the new config,
and the old workers could not be killed by apisix stop

I think we need to fix it by:

1. When running apisix start, Check if APISIX has been running. If is running, remind to use apisix reload

2. apisix reload needs to regenerate nginx.conf according to config.yaml and then reload

3. When running apisix start or apisix reload , check the nginx.conf syntax before truely run the command.

Environment

• apisix version (cmd: apisix version):
  1.3
• OS:
  centos7

Minimal test code / Steps to reproduce the issue

1. apisix start
2. change conf/config.yaml
3. apisix start