pure hashtable is a node type where all keys share a common letter,
so it is consumed on parent node -> pure trie node transition.
So when tracing back, this letter must be returned.

Also updated unit tests to cover these sort of situations.

- reorganized
- some code blocks are split
- SIGALRM handler for main thread

- previously static function 'put_rr' in internet.c has been made public, it handles additions of RRSIGs as well now, by adding the to a list that is traversed after each section has been solved.

- Removed rrset->rdata_indices, RDATA lengths are now stored inline together with RR's TTL
- Added function to get RR's TTL - TTL of the whole RRSet can still be obtained, but it will default to the first RR's TTL or 0
- Renamed few RRSet functions, removed a few too - the file still needs a cleanup and function reordering
- Special handling for RRSIGs during AXFR-out no longer needed (yay)

throughout

- it is as good as the sockaddr_t previously, but standard
- subnets are only needed for ACLs
- common way to print addresses (even AF_UNIX)

- refactored the zone-loader a bit, as reading from zone file
  creates only contents and the zone is created separately
- refactored zone contents creating function, as it is always used in a
  way where an apex node is created and inserted into contents
- crash occured because created zone bound configuration and if parsing
  failed, the configuration was freed with the zone and also with the
  caller

* events now carry pointer to their scheduler
* no event types, events are simple callbacks
* removed unused or deprecated stuff
* bugfix: zone never expired since expire was initialized on create
* bugfix: race condition on rescheduling events
  - there is always some time between cancel and schedule
  - if two threads run run past the cancel and then race for schedule,
    they may rarely schedule an event two times
  - this is hard to deal with unless we place a lock on the zone
    operations, therefore each schedule checks heap if the event is not
    already scheduled and if it is, it replaces the old instance

* removed redundant attributes
* removed unused parameter occurences
* moved related functions to appropriate files
* evsched has now context (server)
* zone does not point to server
* zone event planning is still in the zone loading and requires server #209

also includes removal of knot_ prefixes from private API

close #205

issue #199

Simpler knot_pkt_reserve() API should be used instead.

issue #1