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this prepares cache/txn structures to hold API as well, so we can get
rid of the global api

this fixes a problem when module is unloaded at runtime
and active

the api is still a global variable, but it would be the
best if the namedb_t carried the pointer to the api,
so it needs to change in libknot later

each asset is tagged by a byte defining its type and importance
this is a groundwork for negative cache and packet cache
abstracted the code in preparation for different backends

The pruning is going to be a little tricky with the db,
as of now first query that fills up cache to the brim clears it.

- removed excess copies in query, cache time tracking
- simple insert/replace semantics
- zero-copy query if TTL recalculation is not needed
- doc

this is a little hacked, I'd like to do list of nested
transactions later and short-lived read transactions.

TODO:
pruning, better removal, periodic cleanup, configuration