Resume ballot on top of overlay v2 - #5417

This change allows stellar-core to quickly resume balloting when a
transaction set arrives, rather than waiting for a ballot timeout to
resume.

It creates a registry of stalled slots, and checks tx sets that arrive to
see if they could potentially unstick those slots. A stall can only be
resolved by a tx set if the ballot protocol has not undergone any
changes since hitting the stall point. This prevents prevents us from
resuming with a ballot when the node may have moved on to a different
value.

This change also adds `c` to the calculation of ballot ordering. This is
necessary because without it, a node that exercises this quick-resume
path has no way of informing the network as they will discard a ballot
where the other variables are unchanged.