I haven't tested this in detail but observed the following:
- peer is in sync (blockfilters disabled)
- stop peer
- start peer with
-blockfilters=1 -peerblockfilters - peer immediately sets service flag
COMPACT_FILTERS - filter has just started to sync
Probably not very crucial because AFAIK in case we get a NetMsgType::GETCFILTERS while not having the filter ready, we just don't response.