Handle removed buckets in background persistent config update polling

Removed database configuration documents are handled by SG in 3.0, which closes the associated database once detected.
We don't detect when the entire bucket has been removed, which leaves the database running with DCP stuck in a retry loop, and all SDK operations failing due to the removed bucket.

Proposed Solution

ServerContext.fetchAndLoadConfigs() should iterate over its copy of databases/buckets, and remove any databases that are no longer present in the set returned by the server.