Cancel worker tasks when deleting document

At the moment tasks continue to run and log a lot of`404 Not Found` for the keepalive.

somewhat related: #217

Start by tracing document deletion and the worker tasks that continue running afterward, then inspect how their keepalive requests are handled. Done means deleting a document also stops its associated worker tasks and prevents the repeated `404 Not Found` keepalive logs.

Written by the indexing model from the issue text.