refactor: drop support for etcd v2 API

[tzssangglass]

etcd will soon drop the V2 API: etcd-io/etcd#12913

[membphis]

LGTM

[spacewander]

We don't need to detect the version. The only supported prefix now is /v3, see

lua-resty-etcd/lib/resty/etcd.lua

Line 9 in 70b648f

["3.4."] = "/v3",

[tzssangglass]

change t/v3/tls.t test case 2 response body form

--- response_body
err: 18: self signed certificate

to

--- response_body eval
qr/18: self signed certificate/

Because the step of getting the version is omitted, the self-signed certificate should be returned when getting the version, but it is returned here:

lua-resty-etcd/lib/resty/etcd/v3.lua

Lines 140 to 143 in 70b648f

	if err then
	health_check.report_failure(endpoint.http_host)
	return nil, endpoint.http_host .. ": " .. err
	end

with endpoint.http_host

[membphis]

if we only support v3 protocol, can we drop protocol field? it is useless.

[tzssangglass]

Maybe etcd will have a v4 protocol in the future?

[membphis]

for protocol field, the default value should be v3, all right?

[tzssangglass]

@membphis pls review again, thanks