This makes fold-from-snapshot use the back pressure
model of doing plain_rpc:call to the merge worker
delivering chunks of 100 KVs.
The back pressure is entirely internal to hanoi,
designed to ensure that the process that merges
fold results from the individual levels is not
swamped with fold data.
Folds with a limit < 10 still do "blocking fold"
which is more efficient and uses fewer FDs, but
blocks concurrent put/get operations.
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