net: use `_server` for internal book-keeping

The role of `this.server` is now split between `this._server` and
`this.server`. Where the first one is used for counting active
connections of `net.Server`, and the latter one is just a public API for
users' consumption.

The reasoning for this is simple, `TLSSocket` instances wrap
`net.Socket` instances, thus both refer to the `net.Server` through the
`this.server` property. However, only one of them should be used for
`net.Server` connection count book-keeping, otherwise double-decrement
will happen on socket destruction.

Fix: #5083
PR-URL: https://github.com/nodejs/node/pull/5262


Reviewed-By: James M Snell <jasnell@gmail.com>