RFC 7858 explicitly defines an out-of-band key pinning profile as one
authentication mechanism.  It uses the same format for representing
the pin as HPKP does (RFC 7469).

By logging this pin directly upon first use of the X.509 credentials,
we make it a little bit easier for an admin to publish part of a
pinset.

For ideal operation (including preparation for key rollover), a backup
public key should also be provided, but this is not defined
functionally here.

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