Implement asynchronous query import on startup

[DL6ER]

What does this implement/fix?

Implement asynchronous importing of queries on startup. While the queries are imported, DNS resolution should be available. Overall, this increases the total workload (due to much more locking/unlocking), however, it is less of an issue now as the entire importing process is happening non-blocking in a dedicated thread after starting up.

TODO

• Prevent GC from running until import is done
• Reduce lock/unlock work by blocking in batches of, say, 100 queries (?) to remedy the added workload notably
• Check if we really need pre-allocate the additional space for domains/clients/upstreams (probably not) because we will not see all of them during the subsequent query parsing. We will, however, need the query space.
• Check if we should optimize the shared memory resizing by resizing in units of entire pagesizes. In the end, it will still mean we have less than 1 MB per allocation step and memory is cheap these days

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Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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