hdf5_msb

Avoid leaving a v1 B-tree used as a chunk index in a bad state
that makes assertions fail.

Add an optional `close` method to the `H5D_chunk_ops_t`, and use that to
release "holds" on metadata cache (MDC) entries.

For extensible arrays and v2 B-trees, use the existing `dest`(roy)
method to implement `close`.  For v1 B-trees and other chunk indices,
don't provide `close`: we cannot safely close the v1 B-tree index, and
the other indices don't have a meaningful presence in the MDC.

Revert my first attempt at making v1 B-tree chunk indices closeable
with `dest`.

Put my comment about the stopgap fix for VFD SWMR at the right place
in src/H5Dchunk.c.