Overhaul network activity toggle

- Rename RPC command "togglenetwork" to "setnetworkactive (true|false)"
- Add simple test case
- GUI toggle added to connections icon in statusbar

src/rpc/client.cpp

@@ -107,6 +107,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "prioritisetransaction", 2 },
     { "setban", 2 },
     { "setban", 3 },
+    { "setnetworkactive", 0 },
     { "getmempoolancestors", 1 },
     { "getmempooldescendants", 1 },
 };