CEP-45 - Fix consensus request forwarding bugs and extract it from St…

[bdeggleston]

…orageProxy

• CasForwardResponse held a single-use RowIterator that serialize and serializedSize both drained, so the write pass emitted an empty partition and tripped messaging's length assertion. deserialize had the mirror defect. Materialise a FilteredPartition on both sides.
• Forwarded timeouts arrived nested in an ExecutionException and became bare RuntimeExceptions, erasing the indeterminacy callers judge by exception type. Examine the cause and translate.
• A CAS that applied reports no result, which read as "no forwarding needed", so the caller re-ran it locally. Wrap the outcome in Forwarded.

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[aweisberg]

+1