messager: avoid taking locks if there's no work to do #4745
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In reviewing #4739 I noticed that the txpool calls into the messager engine on every committed transaction with the list of new messages / changed messages.
For all cases in which the transaction did not actually change a messages table, the two lists are empty, but the messager engine still takes internal locks to process these empty lists.
It seems like this could cause unnecessary lock contention, so add an initial short-circuit check in case both lists are empty.