Implement bulk insert on create

[declanvk]

Problem

• When creating a tree from an existing source of key-value pairs, the method of inserting one at a time can be slow.

Solution Sketch

• In section 3F (bottom of page 6 in PDF) of the adaptive radix tree paper, it sketches out an algorithm for "bulk loading":

When an index is created for an existing relation, the following recursive algorithm can be used to speed up index construction: Using the first byte of each key the key/value pairs are radix partitioned into 256 partitions and an inner node of the appropriate type is created. Before returning that inner node, its children are created by recursively applying the bulk loading procedure for each partition using the next byte of each key.

• This algorithm is only applicable when constructing the tree for the first time, this isn't a bulk loading algorithm for an existing tree.