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.

appinfo/info.xml

@@ -41,6 +41,7 @@ See the README for known limitations and the recovery kit before relying on it.]
         <command>OCA\NgBackup\Command\TargetList</command>
         <command>OCA\NgBackup\Command\TargetRemove</command>
         <command>OCA\NgBackup\Command\RunBackup</command>
+        <command>OCA\NgBackup\Command\CancelRun</command>
         <command>OCA\NgBackup\Command\Status</command>
         <command>OCA\NgBackup\Command\ListSnapshots</command>
         <command>OCA\NgBackup\Command\Browse</command>