mem usage & cpu optimization for the delta update handler

[nbittich]

When the /update handler is spammed with delta updates, mu search can't keep up (which makes sense) and consume a lot of memory & cpu, although the updates are cached before being consumed:

[nbittich]

a possible optimization could be to dedup and filter by subjects before adding to the queue:
https://github.com/mu-semtech/mu-search/blob/master/lib/mu_search/delta_handler.rb#L63