add needUpgrade() check to addQueuedTask()

An attempt at preventing
```
"An exception occurred while executing 'SELECT "id" FROM "oc_jobs" WHERE "class" = ? AND "argument" = ?': SQLSTATE[HY000]: General error: 1 no such table: oc_jobs".
```
when adding queued jobs. An addition to https://github.com/owncloud/core/pull/6406 which adds the `needUpgrade()` check to `addRegularTask()`

see related https://github.com/owncloud/core/issues/6313#issuecomment-32582948

@PVince81 @karlitschek @DeepDiver1975 @icewind1991 @schiesbn I tried to reporoduce the oc_jobs error with mysql or postgresql but could not provoke it. Any hints on reproducing it are welcome.