Implement new scheduler (#823)

- introduce internal/router/scheduler to decouple routing, swapping and
queuing into interface contracts.
- introduce a new `routing` configuration section that supersedes
`matrix` and `group` while maintaining backwards compatibility
- add FIFO scheduler with prioritized queuing 
- add internal/router/design.md as developer documentation on
implementing new schedulers and routers

Fixes #797

internal/config/matrix_test.go

@@ -289,7 +289,9 @@ matrix:
 	cfg, err := LoadConfigFromReader(strings.NewReader(yaml))
 	require.NoError(t, err)
 	assert.NotNil(t, cfg.Matrix)
-	assert.Len(t, cfg.ExpandedSets, 2)
+	assert.Len(t, cfg.Matrix.ExpandedSets, 2)
+	assert.Equal(t, "matrix", cfg.Routing.Router.Use)
+	assert.Len(t, cfg.Routing.Router.Settings.Matrix.ExpandedSets, 2)
 	// Groups should be empty when matrix is used
 	assert.Empty(t, cfg.Groups)
 }