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

by default, build system attempts to use LMDB
from the system. however if it's not found or
the version is too old, it uses the built-in
snapshot in contrib

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

thanks @darix!
fixes #21,#22

* 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

this is needed to make sure it always compiles with PIC

RRSets are merged by using stash_add().

1. validate module must be between iterate/cache
2. produce: copy OPT with DO=1, ask for DNSKEY if we don’t have it
3. resolve.c: subrequest DNSKEY if asked to do it
4. consume: check DNSKEY and set it, validate RRSIGs against it

another issues:

rrsigcache is copypasta of rrcache, there is one special case with storing RRSIGs which doesn’t deserve it’s own module (if the validation is off, then nothing will get written in there anyway)

since the resolution is asynchronous, layers must only *ask* resolver to do subrequests for them using query flags (like when we encounter an unknown zone cut)

also some cleanup and version bump to Q2

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

previously cryptolib random function was used to generate message id,
this works well but it is slow especially when the entropy is low,
replaced with cryptographically safe prng ISAAC

the ccan directory is going to be used in the future, as it's include
structure makes it easy to embed C snippets instead of reimplementing
them

interface split to ‘map’ and ‘set’
added documentation
array: added array_tail() operation