Backups to SMB no longer crawl; add occ backup:cancel

Each 30-second step opened the location again and read its list of
index files. Over SMB that cost a request per file, so on a large
repository opening took longer than the step: each step stored one
4 MiB pack and the list grew further. A first backup of nc01 to a
Storage Box over SMB managed 12 GiB in 35 hours.

occ backup:run and system cron now keep the repository open for the
whole run (reopened when another process took a step or the location
was edited), opening no longer counts towards the step's time, and
SMB folders are listed in one request.

occ backup:cancel stops a running backup: no new step starts and the
step in progress finishes first. A run of an earlier version, which
does not hold back its next step, is marked cancelled after two
minutes unless it is recording its snapshot.

README.md

@@ -195,6 +195,7 @@ On the backlog; none of these block the beta or 1.0.0, but know them before you
 | `backup:key:slots`, `backup:key:slot:add/replace/remove` | passphrase slots |
 | `backup:target:backends/add/edit/list/remove/append-only` | backup locations |
 | `backup:run`, `backup:status`, `backup:list` | back up, status, restore points |
+| `backup:cancel <run>` | stop a running backup |
 | `backup:browse`, `backup:restore` | browse and restore |
 | `backup:retention`, `backup:prune`, `backup:trash`, `backup:trash:restore` | retention, cleanup, trash |
 | `backup:verify` | checksum audit of a restore point (`--deep` downloads and decrypts everything) |