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

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 integration tests will run from Python, from here a mock resolution
context is imported, so the Python module can modify time/recursion
endpoints.