gc: separate checkpoint references from wal consumers

Initially, gc_consumer object was used for pinning both checkpoint and
WAL files, but commit 9c5d851d ("replication: remove old snapshot
files not needed by replicas") changed that. Now whether a consumer pins
WALs or checkpoints or both depends on gc_consumer_type. This was done
so that replicas wouldn't prevent garbage collection of checkpoint
files, which they don't need after initial join is complete.

The way the feature was implemented is rather questionable though:
 - Since consumers of both types are stored in the same binary search
   tree, we have to iterate through the tree to find the leftmost
   checkpoint consumer, see gc_tree_first_checkpoint. This looks
   inefficient and ugly.
 - The notion of advancing a checkpoint consumer (gc_consumer_advance)
   is dubious: there's no point to move on to the next checkpoint after
   reading one - instead the consumer needs incremental changes, i.e.
   WALs.

To eliminate those questionable aspects and make the code easier for
understanding, let's separate WAL and checkpoint consumers. We do this
by removing gc_consumer_type and making gc_consumer track WALs only.
For pinning the files corresponding to a checkpoint a new object class
is introduced, gc_checkpoint_ref. To pin a checkpoint, gc_ref_checkpoint
needs to be called. It is passed the gc_checkpoint object to pin, the
consumer name, and the gc_checkpoint_ref to store the ref in. To unpin a
previously pinned checkpoint, gc_checkpoint_unref should be called.

References are listed by box.info.gc() for each checkpoint under
'references' key.