caches.go clientDown may have a memory leak problem

[Allyin]

I discovered a memory leak in a service that constantly creates, looks up, and deletes tables. the code juse delete the client from array but not close the client. the client's goroutine like receive_rpcs(contain one tmp buffer for conn) will always exist. For example, a table is deleted, but regionsever is normal. Requesting again may trigger the shutdown logic, but I'm not sure.