Set the WorkerClient#mIsConnected to false if Thirft exception happen

Set the WorkerClient#mIsConnected to false if Thirft exception thrown when trying to open connection.

This will allow the state to be set correctly if caller need to reuse instance of WorkerClient.
Yep, but it is assuming no other method setting it to true because we are relying on constructor to set the initial value. 
So this chance guarantee that the mIsConnected state is set properly.

The actual reason for this PR is to remove mUserId attribute from WorkerClient because it is only needed when it is opening connection to set heartbeat thread. 
Currently TachyonFS always create new WorkerClient whenever it tries to open connection and a lot of code set TachyonFS's instance of WorkerClient to null to reset it. With this change I would like to only create new WorkerClient at constructor of TachyonFS and pass TachyonFS#mUserId as input argument to WorkerClient#open.

This PR allows an owner of WorkerClient (in this case TachyonFS) to own an instance of WorkerClient instead of discarding it when WorkerClient#open fail. So, in any exception catch we don't need to set instance of WorkerClient to null (i.e. via call to  mWorkerClient = null) but simply to call WorkerClient#close to do proper cleanup.