Fix an issue causing excessive memory consumption.

This commit resolves an issue where the block node index was forcing
entire blocks to be kept in memory thereby forcing excessive memory usage.
For example, prior to this change, the memory usage could consume upwards
of 1.5GB while importing bootstrap.dat via the addblock utility.  With
this change the entire import takes <150MB.  This also has the same memory
reduction to btcd since it uses the same code path.