Side-tag cleanup #88

As a follow-up to #79 : Should we be removing old side-tags when ELNBuildSync is restarted?

During the review of #87 it occurred to me to consider what we should do about leftover side-tags that remain when we restart. My thoughts on that ticket were:

While I was reviewing this, I thought also about whether we should similarly have it clean up old side-tags when it starts up, but I am coming down on the side of NOT doing that, and requiring it to be a human who cleans it up. The reasoning is that if EBS somehow ended up in a crash-loop where the crash occurs after the cleanup, we might not have any way to rescue partially-completed batches manually. So I think it's probably safer if we leave the mess and clean it up periodically by hand.

(I guess we could also try to do something like "clean up all side-tags older than 30 days", but either way we should probably discuss this in a ticket first.)

So this is that ticket: should we clean up old side-tags at EBS process startup? I don't think we necessarily need to be doing it on a periodic basis, but maybe whenever we restart EBS, we should remove side-tags it created over a month ago?

@bhoy-troy @yselkowitz @jsencak