[Payments algo/Reservations] Perform reservations in both sides in one round, instead of two rounds

[HaySayCheese]

For the path

C -> A -> B -> D

if C wants to reserve amounts on the path it would repeat the next action:

C->A: Reserve amount to C
C->A: Reserve amount to A
C->B: Reserve amount to A
C->B: Reserve amount to D

We can improve it in the next way:

C->A: Reserve amount to C AND A
C->B: Reserve amount to A AND D