[master < T981] Reduce all shortest paths memory usage

[antoniofilipovic]

TLDR: Based on the mgbench results, memory usage dropped from gigabytes to a few megabytes, while performance on all shortest paths dropped ~7% -> the decision is to merge because of the significant gain in memory usage while keeping performance high.

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[gitbuda]

@antoniofilipovic can we merge this to master before v2.9 is released (include in the v2.9 release)?

[Josipmrden]

please provide text for the PR message

[Josipmrden]

small comments, overall GJ

[antoniofilipovic]

@vpavicic This PR changes the way memory is used in all shortest-path queries. From now on users should see lower memory usage with query performance almost staying the same.