Remove checkpoint handle code from __checkpoint_worker(), its callers
should deal with checkpoint handles.

Assert in __wt_checkpoint(), __wt_checkpoint_write_leaves() and
__wt_checkpoint_sync() they're never called with a checkpoint handle.

Assert in __wt_checkpoint_sync() that we have a block manager reference
to sync.

__wt_checkpoint_close() is the only place correctly called with
checkpoint handles, where we close WT handles, deal with checkpoint
handles explicitly there.
Merge pull request #728 from wiredtiger/handle-cleanup

Minor handle cleanup/restructuring.