osd: Add asserts to look for potential missing list corruption.

osd:  Add asserts to look for potential missing list corruption.

The missing list implementation relies on some assumptions about
the way in which it is used.  The assumption is that there is a
1:1 mapping between objects and versions. Care must be taken by
callers to ensure that any changes to this 1:1 mapping are done
in such a way as to not corrupt the missing or rmissing list.

This change adds several asserts which police this assumption.

The asserts are being added in a second commit, so the basic
fix can potentially be backported without backporting the
asserts. It is possible that the assert could detect corruption
that otherwise would not have been a problem.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>