Description:
    Check in Vailin's performance improvements:

- Track min+max for v2 B-trees, so we can more quickly answer queries

- Refactor chunk index/hash value computation so that it doesn't have to be
    updated on cached chunks every time the dataset's dimension sizes change.

Tested on:
    Mac OSX/64 10.10.0 (amazon) w/gcc 4.9, FORTRAN, C++ & parallel
    (h5committest not required on this branch)