gc: cleanup garbage collection procedure

Do some refactoring intended to make the code of gc_run() easier for
understanding:
 - Remove gc_state::checkpoint_vclock. It was used to avoid rerunning
   engine gc callback in case no checkpoint was deleted. Since we
   maintain a list of all available checkpoints, we don't need it for
   this anymore - we can run gc only if a checkpoint was actually
   removed from the list.
 - Rename gc_state::wal_vclock back to gc_state::vclock.
 - Use bool variables with descriptive names instead of comparing
   vclock signatures.
 - Add some comments.