Issue #1: Async interface for Btree::insert

Adds the Btree::InsertClosure structure as an interface to insert
data to the Btree. This will replace Btree::insert().
Issue #1: Move Btree::insert() to InsertClosure
Issue #1: Implement Btree::SplitClosure
Issue #1: Implement Btree::FindClosure
Issue #1: Implement Btree::IterateClosure
Issue #1: Implement Btree::SearchNodeClosure
Issue #1: Allow async compare functions