Feature balancer disable ssl

[vislee]

I hereby granted the copyright of the changes in this pull request to the authors of this lua-resty-core project.

ref: https://github.com/openresty/lua-nginx-module/pull/1687

[doujiang24]

@vislee thanks for your contribution. disable_ssl is not a good idea, set_upstream_ttl(enabled) should be better, we can decide http or https per upstream try.

[vislee]

@vislee thanks for your contribution. disable_ssl is not a good idea, set_upstream_ttl(enabled) should be better, we can decide http or https per upstream try.

@doujiang24 Yes, disable_ssl is not accurate, but this feature relies on 'proxy_pass https://'. I'm concerned that set_upstream_ttl(enabled) might be misused.

[doujiang24]

but this feature relies on 'proxy_pass https://'. @vislee yeah, I knew it, we can only clarify it in the document. I think set_upstream_ttl is better since we can support this usage:

set_upstream_tls(false) -- first try
set_upstream_tls(true) -- second try

maybe we should put this api to another new c module? like lua-conf-nginx-module. thoughts? @thibaultcha @spacewander @rainingmaster