Preliminary cleanup for hash index code (doesn't attack the locking problem

Preliminary cleanup for hash index code (doesn't attack the locking problem
yet).  Fix a couple of bugs that would only appear if multiple bitmap pages
are used, including a buffer reference leak and incorrect computation of bit
indexes.  Get rid of 'overflow address' concept, which accomplished nothing
except obfuscating the code and creating a risk of failure due to limited
range of offset field.  Rename some misleadingly-named fields and routines,
and improve documentation.