shill: Assign "Default" status to the best connection

Set the highest-ranked connection to be the "Default".
As a result, the highest priority default route will
move with the highest-ranked connection in the service
list.

Bonus changes:
 * Service now formally holds a reference to the Connection
   object, so call a "SetConnection" method instead of a
   Create/Destroy of the HTTPProxy.
 * Actually start the routing table service, and do a couple
   minor fixes due to how the kernel actually accepts metric
   changes.

BUG=chromium-os:7607,chromium-os:23993
TEST=New Unit Test + Manual (watch routes while inserting
USB-Ethernet on a machine connected to WiFi)

Change-Id: Iddf1ed766238d9e8adc97bb54fc12b527f86239f
Reviewed-on: https://gerrit.chromium.org/gerrit/12685
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Reviewed-by: Gaurav Shah <gauravsh@chromium.org>
Commit-Ready: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
diff --git a/connection.cc b/connection.cc
index 25291b5..460654d 100644
--- a/connection.cc
+++ b/connection.cc
@@ -75,7 +75,7 @@
   }
 }
 
-void Connection::SetDefault(bool is_default) {
+void Connection::SetIsDefault(bool is_default) {
   VLOG(2) << __func__;
   if (is_default == is_default_) {
     return;