Remove server registration from vclock

* Remove vclock_add_server()/vclock_has() - servers are implicitly
  registered in vclock->map on the first request
* As result, lsn == 0 is never exist in vclock
* box.info.vclock[server_id] returns nil instead of 0 even if
  server_id was registered in _cluster, but hasn't sent any rows
* Soften ER_UNKNOWN_SERVER checks for unregistered server_id
* Fix buggy ER_MISSING_SNAPSHOT logic
* Add missing "error.h" includes