Add user-client mapping to facilitate statistics on the usage of each user connection

[Shuen14]

User-client mapping has been added to the server to facilitate statistics on the usage of each user connection.

Additionally, a new client count command has been added to output the connection details of each user.

[CharlesChen888]

LGTM at a glance.

Now that we have a user-client map, maybe try to improve CLIENT KILL USER <username> by using this map instead of traversing through the whole list? Also some of the ACL commands?

[madolson]

We will want the client count command to conform to the following spec: https://github.com/redis/redis/issues/12311, in that it should support similar filtering to client list and client kill.

[Shuen14]

@oranagra could you please take a look? I believe this is a useful PR.

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