possible pool states.  I think it's much clearer now.

Added a new long overdue block-management overview comment block.

I believe the comments are in good shape now.

Added two comments about possible small optimizations (one getting rid
of runtime multiplications at the cost of a new pool_header member; the
other getting rid of runtime divisions and the pool_header capacity
member, at the cost of a static const vector of 32 uints).