bug(healthcheck): the checker name is wrong if APISIX works under multiple processes.

[membphis]

For a specified upstream, the same name should be used to ensure that the checker objects created between different workers can share the same health check result.

https://github.com/iresty/apisix/blob/master/lua/apisix/balancer.lua#L76

For example, if we got the upstream from a Route, then the name should be /routes/{id}; if we got the upstream from a Service, then the name should be /service/{id}.

[membphis]

fixed: #586