Replace Traefik with mproxy

[mproxy](https://github.com/kevinmcconnell/mproxy) is a custom minimal
proxy designed specifically for Kamal.

It has two big advantages over Traefik:
1. Imperative deployments - we tell it to switch from container A to
   container B, and it waits for container B to start then switches. No
   need to poll for health checks ourselves or mess around with forcing
   health checks to fail.
2. Support for multiple apps - as much as possible, configuration is
   supplied at runtime by the deploy command, allowing us to have
   multiple apps share an instance of mproxy without conflicting config.

test/integration/lock_test.rb

@@ -1,6 +1,6 @@
 require_relative "integration_test"
 
-class LockTest < IntegrationTest
+class IntegrationLockTest < IntegrationTest
   test "acquire, release, status" do
     kamal :envify