Move 'bucket.BucketIterator' -> 'connection._BucketIterator'.

Rename to indicate the iterator is an implementation detail of 'Connection'.
Unneeded shim.
Move 'key._KeyIterator' -> 'bucket._KeyIterator'.

The iterator is an implementation detail of 'Bucket'.
Merge pull request #358 from tseaver/refactor-move_bucket_iterator_to_connection

Move 'bucket.BucketIterator' -> 'connection._BucketIterator'.