Prevent/Warn about reindexing cancellation if user calls `graph.close()`

It would be nice if there was a mechanism to detect if there were any reindexing operations happening on a graph and reject a call to `graph.close()` which would close all the graph's open transactions and thus cancel the reindexing procedure. In the very least, it would be nice to add a log message in the detected case, and if it is not trivial or worthwhile given other priorities to add this feature, then perhaps a general log warning or debug upon all calls to `graph.close()`.

Start at the graph.close() entry point and trace how it closes open transactions while reindexing is active. Then inspect the reindexing lifecycle to determine how an active operation can be detected. Done means either rejecting close during reindexing or emitting a log warning when close would cancel it; no files or tests are named in the issue.

Written by the indexing model from the issue text.