HAMMER VFS - Fix assertion when taking snapshot

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>