registry: add comment explaining workflow to make store_keys safe w/o transactions

registry: add comment explaining workflow to make store_keys safe w/o transactions

/*
 * Make the store operation as safe as possible without transactions:
 *
 * (1) For each subkey removed from ctr compared with old_subkeys:
 *
 *     (a) First delete the value db entry.
 *
 *     (b) Next delete the secdesc db record.
 *
 *     (c) Then delete the subkey list entry.
 *
 * (2) Now write the list of subkeys of the parent key,
 *     deleting removed entries and adding new ones.
 *
 * (3) Finally create the subkey list entries for the added keys.
 *
 * This way if we crash half-way in between deleting the subkeys
 * and storing the parent's list of subkeys, no old data can pop up
 * out of the blue when re-adding keys later on.
 */

The workflow is going to be modified to meet this agendain the next commits.

Michael
(This used to be commit 55dd9bdd148fc942e15aacfe9f6b38b1a5c53158)