Fix a comment.
There are really two issues in reconciliation of internal pages: we have
to both avoid pages being written because it messes up the block lists,
but we also have to avoid pages disappearing from under us when we're
writing internal pages.  Compaction has the same problem as checkpoints:
rework the code so checkpoint and compaction are almost the same code.

This means __wt_sync_file() does all the work, it's no longer called
twice from __wt_bt_cache_op(), (actually a simpler approach than
before).  Merge SYNC_INTERNAL and SYNC_LEAF into SYNC_CHECKPOINT.

Rename btree->checkpointing to be btree->internal_lockdown, it's now
used by compaction.
Merge branch 'develop' into compact-checkout