Reverse DNS Processor should cache failed lookups

## Feature Request

@ssoroka

### Proposal:

Currently, the reverse DNS processor only caches successful lookups. It should also cache failed lookups for a period of time

### Current behavior:

Only successful lookups are cached

### Desired behavior:

Failed lookups would also be cached so that it doesn't constantly try to lookup the same IP

### Use case:

If a device sends a large amount of data to Telegraf, but the IP doesn't resolve using reverse lookup, Telegraf will try to resolve that IP every time a new message comes in. It should remember that it failed and only retry after a defined period of time.

Start by locating the Reverse DNS Processor entry point and its existing successful-lookup cache. Check nearby tests, if present, to understand cache expiry behavior. Done means failed lookups are cached, repeated requests avoid immediate retries, and retries occur after a defined period.

Written by the indexing model from the issue text.