Reloading meshes on edits does not follow subsequent user interations

[ceesem]

It can take several seconds (at least) for edits to go through the pychunkedgraph. During that time, a user can deselect a neuron that is being merged/split, request a new edit, or make other changes that affect what root ids are selected in the scene. However, Neuroglancer does not respect these user events when handling the response from the chunkedgraph edits. For example, if I make a merge and deselect both objects, the subsequent merged root id will become selected even though it no longer replaces anything in the state. Similarly, making a second split edit (on a different object) will often cause an inconsistent mesh loading, often not actually doing the replacement expected.