Siapath to uid
This MR changes the key in the siafilemap to the uid of the siafile.
I'm not sure that this gives us a significant advantage over using the siapath. The advantage of using the UID is being able to temporarily have multiple files with the same siapath in memory after rapidly renaming/deleting/adding files. For constant time lookups in the cache of already loaded files we still need a mapping from siapath to UID which eliminates that advantage unless we map from siapath to a map/list of UIDs.
I'd appreciate some feedback on this proposal.
Closes #3313 (closed)