Lock view page does not show current lock state correctly

[anjackson]

The lock view page relies on the hash map stored under the session, but the actual locks are stored in separate keys. e.g. lock:web/19950418155600/http://portico.bl.uk/ = _SESSION_ID_.

When the latter timeout, that's all fine, and the lock is released to an item can be viewed by another.

However, the UI shows the web/19950418155600/http://portico.bl.uk/ as an entry in a hash set stored at the key SESSION_ID, and hash set entries can't have their own expiration. So, the _locks UI does not show the true locks state.

It's not terribly performant, but given we don't expect that many users, it would probably be fine to scan all locks and group them by session ID in memory, and show that instead.

Current session scanner is at

ukwa-pywb/ukwa_pywb/ukwa_app.py

Line 342 in 153860d

for sesh_key in session.redis.scan_iter(SESH_LIST.format('*')):

[anjackson]

See also #76

[anjackson]

Okay, modified to scan for locks and expire/drop ones that are gone.