Vector clock merge on receive

When a replica receives a message, it merges the incoming vector clock with its own by taking the componentwise maximum, then increments its own entry by one to account for the receive event. Given the local node's id `me`, the local vector clock `local` (a dict; missing node = 0), and the incoming vector clock `incoming`, return the merged-and-incremented vector clock as a dict that includes every node mentioned in either clock plus `me`.