implement Connection.lastModifiedImpl

Here's an implementation of Connection.lastModifiedImpl that stores a "stat" structure (whose only property so far is "mtime") in asyncStorage at the location "!" + path. Suggestions for improvements to how we store the structure are welcome! I'm also not very happy with the callback hell in fstests.js. Maybe those should be a sequence of separate test functions.
Could you add a comment here explaining why you had to modify this?