Small refactors for TorProcess

We could do some small refactors on the TorProcess class:

• use actual ES private stuff (#)
• we could move the password hashing in it, since it's used only to start a process
• we have many calls to this._args.push in two lines: one for the key and one for the value. We could group together, to make them more readable and quicker to understand
• _torProcessStartTime isn't used (it was intended to be used by TorMonitorService, but it got lost during the rafactor of September)
• we could export the exit code of the process when it exits
• onExit and onRestart don't need to be null: they can be empty callbacks, instead
• we could remove the dependency on TorProtocolService, and take the options in the constructor, instead