When the master sees a burst of updates (perhaps sustained), the
slaves "I_HAVE" messages can fall behind the version we've already
sent, and the unpatched code would retransmit already sent diffs!
This can result in substantial amplification (in a local test, 3000
ops turned into 427,000 ops). Though the number of *messages* sent
was actually somewhat smaller, the ever growing message size
ultimately leads to failure.
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