fs/cache: add --fs-cache-expire-duration to control the fs cache

This commit makes the previously statically configured fs cache configurable.

It introduces two parameters `--fs-cache-expire-duration` and
`--fs-cache-expire-interval` to control the caching of the items.

It also adds new interfaces to lib/cache to set these.

lib/cache/cache_test.go

@@ -100,7 +100,7 @@ func TestPut(t *testing.T) {
 func TestCacheExpire(t *testing.T) {
 	c, create := setup(t)
 
-	c.expireInterval = time.Millisecond
+	c.SetExpireInterval(time.Millisecond)
 	assert.Equal(t, false, c.expireRunning)
 
 	_, err := c.Get("/", create)
@@ -127,6 +127,31 @@ func TestCacheExpire(t *testing.T) {
 	c.mu.Unlock()
 }
 
+func TestCacheNoExpire(t *testing.T) {
+	c, create := setup(t)
+
+	assert.False(t, c.noCache())
+
+	c.SetExpireDuration(0)
+	assert.Equal(t, false, c.expireRunning)
+
+	assert.True(t, c.noCache())
+
+	f, err := c.Get("/", create)
+	require.NoError(t, err)
+	require.NotNil(t, f)
+
+	c.mu.Lock()
+	assert.Equal(t, 0, len(c.cache))
+	c.mu.Unlock()
+
+	c.Put("/alien", "slime")
+
+	c.mu.Lock()
+	assert.Equal(t, 0, len(c.cache))
+	c.mu.Unlock()
+}
+
 func TestCachePin(t *testing.T) {
 	c, create := setup(t)