Service subscription counters (totalServiceSubs, serviceSubsCount,
ntfServiceSubsCount) are TVar (Int64, IdsHash). modifyTVar' only forces
the pair to WHNF, so `n +/- n'` and `idsHash <> idsHash'` stay
unevaluated and accumulate an unbounded thunk chain under subscription
and delivery churn (the IdsHash chain also retains a bytestring per
update) - a space leak proportional to the number of updates.
Force both components in addServiceSubs/subtractServiceSubs. Verified
with the load bench: svc churn drops from +5.4 KiB/iter (linear) to flat.