MB-12978: fix crash in fdb_get_* from snapshot on multiple kv stores

Copying the old incoming handle into the snapshot was
causing a read of freed memory if the old handle was closed
Fix by copying over the necessary fields such as kvs_config
into the snapshot handle itself.
Also fixes several other crashes due to read of txn on snapshots
Change-Id: I4bf4dda019e163fb8cc6eed2f9fc586fc1f38b5f