Controller used to provide cost estimates to optimizer about secondary index data access. Implements the StoreCostController interface for the B-Tree index implementation. The primary use of this interface is to provide costs used by the query optimizer to use when choosing query plans. Provides costs of things like fetch one row, how many rows in conglomerate, how many rows between these 2 keys. Note most work of this class is inherited from the generic btree implementation. This class initializes the top level object and deals with locking information specific to a secondary index implementation of a btree.