Fix double free in merge-left rebalance.

This commit fixes a bug where deletions that caused merge-left rebalances were updating
the parent node which caused a node to "reappear" even after it had been deleted. This was
fixed in merge-right rebalances a while ago but merge-left is less frequent so it was
missed until now.

Many thanks to Jordan Sherer (@jsherer) for finding and reporting the bug.
Merge pull request #185 from benbjohnson/fix-bulk-delete

Fix double free in merge-left rebalance.
Fix last element seek.

This commit fixes a bug with seeks where seeking to a key between the end
of a page and the beginning of the next page causes it to return nil. This
was fixed by calling Cursor.next() when the cursor ends up between pages.

This also changes Cursor.Next() so that calling Next() at the end of a bucket
leaves the cursor at the end instead of zeroing out the stack.

Fixes #186.

/cc @PreetamJinka