New zone API

Here's what I imagine:

• global singleton zonedb-like object
• two ways of getting the zone: read / write. Only one thread can write, this should be handled by zone serialization. User will get a handler a will only be able to modify/read the zone via the handler.
• merge zone_t and contents
• No RCU protecting zones anywhere except inside the zone API itself
• Journal operations handled by the API, together with changeset application.
• Only two operations for manipulating the actual data: Add RR/ Remove RR
• Adjusting on-the-fly (might not be possible)
• Semantic checks part of the API, also CNAME checks and co. with possibility to set fail/ignore.
• Sorted changesets, may be used for 'limbo' zone state (i.e. see modifications to zone you've just made)
• Possibility to mmap zones?
• Get rid of RCU (use trie + changesets for now, implement hazard pointers for cleanup)

Steps:

• Reuse RCU and old journal to create the API, the API should not change after this is done. Port the server code to this API
• Drop RCU
• Drop journal
• mmap
• ???
• PROFIT