Asynchronous Vandelay Imports
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Vandelay imports are now monitored from the browser client asynchronously,
meaning the client requests updates from the server instead of waiting for 
the server to respond to the original import request.  This changes allows 
for incremental progress updates in the browser client.

New Database Table
++++++++++++++++++

This adds a new database table vandelay.session_tracker for tracking
in-progress vandelay upload activity.  A new tracker row is added for
each of "upload", "enqueue", and "import" actions, linked for a given
session by the value stored in the "session_key" field.

The table tracks other potentially useful data, like the staff member
and workstation where the action was performed.

Upgrade notes
+++++++++++++
Users of NGINX as a reverse proxy may need to set a suitable
`client_max_body_size` value in the NGINX configuration so that large
MARC record uploads are not truncated. Note that this would have
always been necessary, but since this feature allows larger files
to be more reliably queued and imported, the need to set `client_max_body_size`
became more apparent.