[signer] Add a configurable timeout to signers for how long to wait for the last block of a tenure to be accepted before allowing its reorg

[jferrant]

It is very possible at tenure boundary that signers can enter into a state of limbo due to poorly time block commits relative to the prev tenures last block proposal. To avoid this chainstall, we enable the reorg of the prior locally accepted block. However, there are times where the prior block WOULD reach global acceptance but its just a little delayed. In which case we actually reorg a valid block that if we just waited a bit the chain would not be stalled. Add a timeout to minimize this reorg of a globally accepted block happening. If a proposal does not get globally rejected/approved within x amount of time at the tenure boundary, time it out and start signing a sibling block from the new tenure instead. This will minimize the reorg of a GLOBALLY accepted block. Default it to 30 seconds.