hdf5_msb

Extract a subroutine that closes chunk indices,
H5D__chunk_index_close(), and call it in H5D__chunk_read() after reading
a chunked dataset.  In this way, indices based on extensible arrays and
v2 B-trees do not leave pinned/tagged entries in the metadata cache that
we cannot evict/refresh when we load changes from the shadow file.

Make some changes to the v1 B-tree code that set the pointer to the
closed B-tree to NULL and, further, tolerate a NULL pointer where
previously that was impossible.