No way to rescan cached filters, without retrieving them again

[optout21]

Preamble: It's possible that there is a way just I could not find it, or there is some fundamental reason why this is not possible.

I could not use nakamoto in a way, that when blocks and filters are already downloaded and cached, it re-verifies the filters, without downloading them again.

Use cases:

• Restart client after some time, and reevaluate/reload loaded client (rescan for transactions)
• Having a client with cached data, load and scan another wallet, without re-downloading all the filters.

I observed the following phases, when starting the client:

• loading: blocks (and filters?) are read from disk
• filter retrieval is initiated, from the beginning or the provided hint block height, even if they were previously downloaded
• filters are checked.

Re-downloading the filters can takes some time (10-20 minutes).

In the code, I found these relevant parts:

• client Handle rescan() (handle.rs)
• Filter Manager rescan() (cbfmgs.rs), calls into
• Filter Manager get_cfilters() (cbfmgs.rs).

I suggest to expose the functionality of re-scanning cached filters.

Any comments appreciated.

[blakebyrnes]

@optout21 Did you see this happening over the wire? Perusing the code for something else, the logic in cbfmgr.rs#get_cfilters calls rescan.rs#requests which seems to intend to skip anything in cache.