tree-insert optionally takes [key value] as a sible argument to make it easier to use with reducers
tree-lookup is a placeholder method to actually get data out of trees
first cut at building a btree keyed on message-id
typo
make our btree node a defrecord, in the hope it will make it better for use with reducers
num-nodes-in counts the nodes in a btree.  Name subject to change, clearly
tree-slice returns the values of all nodes between two keys