HAMMER VFS - Fix assertion when taking snapshot
* hammer_ioc_add_snapshot() issues an ASOF lookup for the snapshot and
then a non-ASOF insertion (insertions never use ASOF). However, the
ASOF lookup can modify the cursor's key (cursor.key_beg).
This mismatch between the cursor's key and the leaf being inserted can
then result in an assertion in the btree insertion code.
* Reloading the key before doing the insertion fixes the problem. Also
document the case.
Reported-by: Stathis Kamperis <ekamperi@gmail.com>