Fix slack weigth mismatch between gpl routability and timing driven modes

[gudeh]

Routability mode usually performs "reverts," where it goes back to a saved snapshot of the placement position of cells. When this happens, the slack weights calculated by the timing-driven mode are not reloaded.

To fix this, we need to save the slack weights together with the snapshot and reload the weights when a revert occurs in routability mode.

[maliberty]

Please give more of a description of this issue.

[gudeh]

Currently we do not overlap the routability and timing-driven overflow thresholds to avoid this and other potential issues on the interactions between the two modes.