Flow bindings: Directory layer should support snapshot reads

This would allow us to inspect directories without adding read conflict ranges, which might be something someone would want to do.

In some sense, this is the root cause of #157. However, that issue points to a bug in the tester that is causing spurious errors. Fixing this issue would give us #157 basically for free, but this somewhat of a separate issue.

Start by locating the Flow bindings' Directory layer and reviewing how directory reads currently create read conflict ranges. Compare the behavior described here with issue #157; done means directory inspection supports snapshot reads without adding read conflict ranges.

Written by the indexing model from the issue text.