I don't know why it passed on Linux without any warning.

Fixes https://gitlab.labs.nic.cz/knot/resolver/issues/93.
API of a KR_EXPORT function is changed, so ABIVER is bumped.

fixed incorrect tagging in rrmap where secure rank would overflow

found by @gdemidov

this change introduces new API for cache backends,
that is a subset of knot_db_api_t from libknot
with several cache-specific operations

major changes are:
* merged 'cachectl' module into 'cache' as it is
  99% default-on and it simplifies things
* not transaction oriented, transactions may be
  reused and cached for higher performance
* scatter/gather API, this is important for
  latency and performance of non-local backends
  like Redis
* faster and reliable cache clearing
* cache-specific operations (prefix scan, ...) in
  the API not hacked in
* simpler code for both backends and caller

this is not going to be backwards compatible change, but it will be the first tagged libknot release sufficient for resolver

* PIE,RELRO+NOW and other security features enabled
* support for both static/dynamic builds with BUILDMODE
* dynamic library is ABI-versioned, starting at 1
* pkg-config file is installed

for pktcache same or better rank is required (because it’s a direct answer)
for rrcache better rank is required (unless doing write-through)

for both cases, no cache rank check is needed when inserting secure data

security note: this mitigates possible non-auth NS hijacking

minimised nodata/nxdomain can now be cached if it is
authoritative. also pkt/sec cache are now shared using
the ‘rank’ field to replace insecure version with secure
if needed

the key is now stored in a format friendly to prefix search, the values
also contain one more 16bit field to store rank of the data (to be
utilised later)

fixes potential cache key oob write

the cache_peek() api was reworked to return an error code instead, from this the caller can tell whether the record is present (but expired) or missing. this save a secondary CNAME lookup in case the original record was just expired

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

the pktcache introduces two cache tags PKT/SEC for
basic/DNSSEC responses and stores negative answers in
the cache

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