Blocklist never prunes missing tenants

[joe-elliott]

Describe the bug
If a tenant disappears (say by deleting all of its blocks manually from the backend) the internal in memory blocklist will never be updated to indicate this.

To Reproduce
Steps to reproduce the behavior:

1. Send spans to tempo
2. Let some blocks build up
3. Stop sending spans to tempo
4. Delete the tenant level folder on the backend
5. Note that the tempodb_blocklist_length value doesn't go to 0.

Expected behavior
Blocks should be pruned from the in memory blocklist.

Environment:

Additional Context
This is b/c the pollBlocklist method in tempodb updates the blocklist of any tenant it finds, but if it fails to find the tenant it does nothing.

[dgzlopes]

Hey :)

The blocks should be pruned from the in-memory compactedBlockLists too?