#135 #141 3 event related fixes (deduplication bug, configurable deduplication lookback, and larger handler size)

Fix 1) #135 update events' expiry date on deduplication
Previously events would age out even when being de-duplicated
frequently, resulting in spurious identical events making their way to
the next agent in the pipeline. This isn't the most efficient code (same
as previously), but not worth optimizing further until we've decided
what to do with a more generic deduplication module

Fix 2) #141 change delayed_job.handler to mediumtext (16MB) like event.payload
This isn't a complete fix because if the event.payload is (say) exactly
16MB then the handler will be >16MB and the same problem will occur.
I'll push this in for now though, in practice it means it's much less
likely to occur.

Fix 3) #135 set deduplication "look back" to be configurable (and default to a large number)
I think this is preferable default behavior? Potential performance
issues should be addressed via an efficient deduplication library using
indexed DB fields