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

amalgamated build concatenates all files into a single .c file to
allow compiler see all symbols and produce possibly smaller code.
for binary distributions this is what you want, as it's faster but
may consume more memory during compilation.
it however cannot do incremental builds.

current processed query is always in `request->current_query`

the selected address is now an array with selection, caller can then send the same query to multiple offered targets

refs #35

previously, debug messages were optional with -DWITH_DEBUG
now the debug messages are built in (unless compiled with -DNDEBUG), but
disabled by default

verbose output can be enabled by '-v' or '--verbose' CLI option
or interactively by 'verbose(true|false)' (or in config)

this is a small step for me, but a huge step for resolver

if the query is satisfied from cache, it doesn’t need a zone cut lookup, so it may be deferred until an outbound query is about the be issued.

the resolution has to start from the root for zonecut resolution, otherwise it might stall resolution if the only good NS drops out of cache and only the bad remain

the reason is that LMDB allows only 1 write transaction
per thread, and this doesn’t work well with multiplexed
requests. there is probably going to be some performance hit,
but now transactions are scoped for each cache operation

instead of keeping just random NS and address,
it keeps a map of nameservers => {addresslist}

this allows scoring and correct NS election, fallback
and evaluation

this fixes resolution of multi-level queries, when a side query
encounters a nameserver that requires address resolution

- cache is now fully optional, resolution works without cache as well
- proper mempool per each request, not mixed
- zone cut resolution doesn't need any per-context memory
- now it's possible to do qname minimization
- rplan keeps tabs on resolved queries
- added documentation etc.