Change file opening to on-demand extent lookup

[DissectBot]

Opening a large fragmented file (several TB) takes several seconds. This is because the extent lookup is done ahead of time. This should be changed to lookup extents based on requested offset.

I knew this was going to be a performance issue when i wrote it the way it is and I had hoped to be able to postpone this for a bit longer, but unfortunately this is already becoming a performance constraint.