fs: deglobalise the config #4685

This is done by making fs.Config private and attaching it to the
context instead.

The Config should be obtained with fs.GetConfig and fs.AddConfig
should be used to get a new mutable config that can be changed.

fs/config_test.go

@@ -0,0 +1,29 @@
+package fs
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetConfig(t *testing.T) {
+	ctx := context.Background()
+
+	// Check nil
+	config := GetConfig(nil)
+	assert.Equal(t, globalConfig, config)
+
+	// Check empty config
+	config = GetConfig(ctx)
+	assert.Equal(t, globalConfig, config)
+
+	// Check adding a config
+	ctx2, config2 := AddConfig(ctx)
+	config2.Transfers++
+	assert.NotEqual(t, config2, config)
+
+	// Check can get config back
+	config2ctx := GetConfig(ctx2)
+	assert.Equal(t, config2, config2ctx)
+}