Reduce the number of times we query all mount points

urlIsFastLocalFile is called a lot and every time we query all the mountpoints to
determine whether the file is on a slow one.

We can optimize this based on two assumptions:
- A folder is on only on one mountpoint
- The next call to urlIsFastLocalFile is likely for a file in the same folder

Keep track of the last folder and its mountpoint and only query the mountpoints
when the folder changes