Halt at blocktime rather as an alternative to halt at block height

[zmanian]

@mdyring suggested that it would facilitate easier upgrades if we had functionality to halt a node at any blocktime greater than a target time.

This would enable a simpler upgrade procedure that would reliably target a specific upgrade time rather than a block height which is difficult to reliably target weeks out.

[alexanderbez]

This could certainly be useful! I propose we still keep halt-height but also introduce halt-time as you've outlined. As a side note, we also need to fix the corresponding logic to not exit directly, so outside defers can run:

	defer func() {
		halt := app.haltHeight > 0 && uint64(header.Height) == app.haltHeight
		halt = halt || (app.haltTime > 0 && header.Time.UnixNano() > app.haltTime.UnixNano())
		if halt {
			app.logger.Info("halting node per configuration", "height", app.haltHeight)
			_ = p.Signal(os.Interrupt)
		}
	}()