Dont issue DEL if cache miss

[ckcr4lyf]

From some naive tracing on rfc5746.mywaifu.best, we have an announce:

Though there was a cache miss, we still issue a DEL to invalidate (a nonexistent) cache. We can probably avoid this wasted command.

[ckcr4lyf]

Additionally: there may not be much value to SET cache for low popularity torrents (save memory).

TBD: What defines popularity