lib ldb key value: fix index buffering

lib ldb key value: fix index buffering

As a performance enhancement the key value layer maintains a cache of
the index records, which is written to disk as part of a prepare commit.
This patch adds an extra cache at the operation layer to ensure that the
cached indexes remain consistent in the event of an operation failing.

Add test to test for index corruption in a failed modify.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>
Signed-off-by: Andrew Bartlett <abartlet@samba.org>