Add optimized initial block index load function.

This commit adds a new optional function named GenerateInitialIndex.  It
generates the required number of initial block nodes in an optimized
fashion.  Since the memory block index is sparse and previous nodes are
dynamically loaded as needed, this function is not stricty needed.
However, during initial startup (when there are no nodes in memory yet),
dynamically loading all of the required nodes on the fly in the usual way
is much slower than preloading them.