Add deletion support to B+ tree

[kevmo314]

We want the ability to delete records from the B+ tree. To do this, we will tombstone leaf nodes.

Because the B+ tree can store multiple values per key, we will delete by key/value pair. But crucially, we will match the values by offset only. This is pretty subtle so it should be documented carefully. That is, our function signature should look like:

func (t *BPTree) Delete(key []byte, valueOffset uint64) (bool, error) {

The algorithm for deleting will be as follows:

1. Find the first node matching key. This is identical to Find today.
2. Call .Next() for each matching key, update the leafPointer to have offset ^uint64(0) (ie tombstone it).
3. Update .Next() to skip tombstoned entries.