Connect to Tor hidden services by default

Adds 127.0.0.1:9050 for the .onion proxy if we can succesfully connect
to the control port.

Natural followup to creating hidden services automatically.

doc/tor.md

@@ -100,8 +100,9 @@ Zcash has been updated to make use of this.
 
 This means that if Tor is running (and proper authorization is available),
 Zcash automatically creates a hidden service to listen on, without
-manual configuration. This will positively affect the number of available
-.onion nodes.
+manual configuration. Zcash will also use Tor automatically to connect
+to other .onion nodes if the control socket can be successfully opened. This
+will positively affect the number of available .onion nodes and their usage.
 
 This new feature is enabled by default if Zcash is listening, and
 a connection to Tor can be made. It can be configured with the `-listenonion`,

src/torcontrol.cpp

@@ -449,6 +449,15 @@ void TorController::auth_cb(TorControlConnection& conn, const TorControlReply& r
 {
     if (reply.code == 250) {
         LogPrint("tor", "tor: Authentication succesful\n");
+
+        // Now that we know Tor is running setup the proxy for onion addresses
+        // if -onion isn't set to something else.
+        if (GetArg("-onion", "") == "") {
+            proxyType addrOnion = proxyType(CService("127.0.0.1", 9050), true);
+            SetProxy(NET_TOR, addrOnion);
+            SetReachable(NET_TOR);
+        }
+
         // Finally - now create the service
         if (private_key.empty()) // No private key, generate one
             private_key = "NEW:BEST";