vchoi_fork

In H5FD_vfd_swmr_read(), do not verify checksums on shadow entries
that are longer than the buffer that the caller supplied: the checksum
usually will fail, but that's not actually a fatal condition, and
usually we will have another opportunity to verify the checksum.

In H5FD_vfd_swmr_read(), remove a bunch of disused code.

In H5FD_vfd_swmr_read(), do not re-read a shadow image that has a
bad checksum, because a bad checksum indicates a serious problem
(writer outran reader, OS defect, hardware failure) from which
H5FD_vfd_swmr_read() cannot recover.

Rationale: the writer write(2)s new shadow images before the new index,
and the new index before the new header.  In H5FD_vfd_swmr_read(),
the reader has read(2) both the index and the header in full.  POSIX
semantics indicate that in these circumstances, the last shadow image
write(2) MUST be completely visible when we read(2).  That is, the index
write(2) & read(2) and the header write(2) & read(2) pair cannot
divide a preceding shadow-image write(2).

The reader may see a "torn" image at this juncture if, for example,
the writer got max_lag ticks ahead of it and reused the storage for
this shadow image.  Even if the reader "recovers" by re-reading the
image until its checksum is correct, it cannot be sure that the
image thus read is the right one for the HDF5 address passed to
H5FD_vfd_swmr_read(), and it cannot be sure that the image thus read is
not stale, because it's operating with an out-of-date shadow index.

Add log outlets swmr_read, swmr_read_exception, and swmr_read_err.
Log to `swmr_read` on entry to H5FD_vfd_swmr_read(), log to
`swmr_read_exception` when checksums are skipped for exceptional
conditions (page buffer not configured, buffer shorter than shadow
image), and log to `swmr_read_err` when the checksum fails.