NEXUS-5472: Last run results were overriding current state

If discovery was in progress, it was not shown on UI
as persisted "last" results were simply hiding it.

Now the resource tells about currently running remote
discovery, even if there was last status. Last status
is used only in case when there is no currently running
discovery.
NEXUS-5578: When remote is scraped, prefix file is removed on discovery

This was an non handled side effect how prefix file discovery
worked, as it tried to get the prefix file (not present remotely
as we got last prefix file by scraping), and that meant that
scraping started with prefix file removed (by prefix strategy).

Now handling this case