Currently (5030), by design or accident, bottlenecks don't occur. If stars are connected by lanes: A -- B -- C, and i build up my forces in B, my enemies are not inconveninced. They can simply chart a course from C to A, and no battle at B will occur, even if both are hostile.
This is not quite what's happening in game, as far as my tests show. Presently, if there is an aggressive enemy armed fleet in a system, your fleets that are ordered to move through the system are forced to stop and end their turn's movement in that system. This will prompt a battle if your fleets are visible to the enemy empire. You can continue moving any ships that survive the battle, but that's intentional; the idea is that you fought past them. It's quite possible for arriving fleets not to survive to the next turn in a system containing hostile fleets, though.