Modify ProposalAllocator to allow specification of max inflight requests at construction.
Allow specification of max inflight requests for PacketSorter.
Make sure default max inflight is the same across PacketSorter and ProposalAllocator.
Revise LeaderFactory to handle leader pipelining with max-inflight leaders being dictated by ProposalAllocator.
Remove superfluous todo (now ProposalAllocator limits active leaders rather than LeaderFactory).