Add support for refreshing circuits ("new identity")

• This commit adds support for the equivalent of Tor Browser's "new identity" button, both globally and per-app.
• Calling refreshCircuits() from Java will terminate all currently open TCP sockets, and cause new TCP connections to go over freshly established circuits.
  • A refreshCircuitsForApp() method allows this process to be restricted to just one app UID.
• Under the hood, this uses a new "command stream" scaffolding API: a TunnelScaffolding can return a Stream of TunnelCommands that lets the scaffolding asynchronously request a circuit refresh.
  • This mechanism can trivially be extended to other kinds of asynchronous request, too.
  • Stopping the proxy can now be achieved by just dropping the command stream (if there is one), which we make use of.
• Note that we currently don't get any confirmation back from the proxy about this being successful, although arguably this isn't really needed.
  • The Java API will now throw a new exception, ProxyStoppedException, if this command or the stop command fails to execute because the proxy is stopped.
  • This helps guard against a poorly programmed application getting out of sync and giving a false sense of security to the user.
• A new floating action button in the sample app lets you test out this functionality globally.
  • I couldn't test the per-app version yet, because that requires more Android knowledge than I currently have!

Fixes #48 (closed).