Add persistence for torperf/onionperf

As discussed in #21272 comment:18 and before.

All storing of files should be done by the persist-mechanism, i.e., a o.t.c.persist.TpfPersistence class needs to be introduced (this would shorten TorperfDownloader quite a bit and prevent another re-implementation of this functionality).

Decide if sync-functionality for the Xperf files can be done with this ticket, too, or needs another ticket.