This commit fixes an issue where keys are skipped by cursors after
deletions occur in a bucket. This occurred because the cursor seeks
to the leaf page but does not check if it is empty.
Fixes#429, #450
This commit changes the API for:
Tx.CreateBucket()
Tx.CreateBucketIfNotExists()
Bucket.CreateBucket()
Bucket.CreateBucketIfNotExists()
These functions now return the *Bucket and error instead of just the error.