Textures currently stored in a dynamic atlas do not get updated when they are reimported from the cache server.

Steps to reproduce:
1. Install a cache server (accelerator)
2. Unzip the provided project, and make sure that the images under the Dirty directory differ from those under the Clean directory. Re-save them if they're identical. Otherwise the asset database may think that they're identical.
3. Open the provided project, and setup the cache server settings
4. Replace the textures in Assets with those in the Dirty folder, and force a reimport
5. Replace the textures in Assets with those in the Clean folder, and force a reimport
6. Close Unity
7. Reopen the project
8. Open the metrics page of the cache server, and note the value of "Data Served from cache" (it should increase after step 10)
9. Back to Unity, open the window in Bug/TextureImport
10. Replace the textures in Assets with those in the Dirty folder through the explorer
11. Go back to Unity

Expected result: The window should now display the "dirty" textures
Actual result: The window still displays the "clean" textures