fix(traffic-split): configure multiple "rules", the request will be confused between upstream

[Firstsawyou]

What this PR does / why we need it:

When the rules array object in the traffic-split plugin has multiple upstream conditions (weighted_upstreams + match), the request will be confused among multiple upstreams. The following example:

curl http://127.0.0.1:9080/apisix/admin/routes/1 -H 'X-API-KEY: edd1c9f034335f136f87ad84b625c8f1' -X PUT -d '
{
    "uri": "/hello",
    "plugins": {
        "traffic-split": {
            "rules": [
                {
                    "match": [
                        {
                            "vars": [
                                ["http_id","==","1"]
                            ]
                        }
                    ],
                    "weighted_upstreams": [
                        {
                            "upstream": {
                                "name": "upstream_A",
                                "type": "roundrobin",
                                "nodes": {
                                    "127.0.0.1:1981":1
                                }
                            },
                            "weight": 3
                        }
                    ]
                },
                {
                    "match": [
                        {
                            "vars": [
                                ["http_id","==","2"]
                            ]
                        }
                    ],
                    "weighted_upstreams": [
                        {
                            "upstream": {
                                "name": "upstream_B",
                                "type": "roundrobin",
                                "nodes": {
                                    "127.0.0.1:1982":1
                                }
                            },
                            "weight": 3
                        }
                    ]
                }
            ]
        }
    },
    "upstream": {
            "type": "roundrobin",
            "nodes": {
                "127.0.0.1:1980": 1
            }
    }
}'

Test :

Match the upstream of the 1981 port and return the 1981 port number:

for i in `seq 1 5`; do curl http://127.0.0.1:9080/hello -H 'id: 1'; done
1981
1981
1981
1981
1981

Matches the upstream of the 1982 port, and is expected to return the 1982 port number, but it actually hits the upstream of the 1981 and 1982 port：

for i in `seq 1 5`; do curl http://127.0.0.1:9080/hello -H 'id: 2'; done
1981
1981
1982
1982
1981

Does not match the upstream of the plugin, the upstream of the default route, returns the 1980 port number:

for i in `seq 1 5`; do curl http://127.0.0.1:9080/hello -H 'id: 4'; done
1980
1980
1980
1980
1980

This PR is used to fix the problem.

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[Firstsawyou]

Related issues: #4090

[tokers]

The PR title is too long, @Firstsawyou could you simplify it?

[Firstsawyou]

The PR title is too long, @Firstsawyou could you simplify it?

Ok, updated.