Fix loader unknown TX stat bug

[simonhf]

This is a hacky fix to get the latest loader code to not report bogus unknown TXs. Although this fix "works"... am wondering if there is a better way? There is a small discussion here [1] and this PR was requested as a way of commenting on the hacky fix and/or possible better / alternative fix :-)

[1] https://axiomzen.slack.com/archives/C024LNQE5N1/p1658201790504759

[Kay-Zee]

yeah, I agree, i think currently, it's possible for the access node to still be waiting for collections itself, so cannot yet serve up the collection to the client until it's received it, in that scenario, then i think this PR isn't that hacky.