SHAMap performance (RIPD-434)

Commit from previous pull request with updates thanks to suggestions given in that pull requests. Updates and bugfixes to address the missing node bugs:

More separation between snapshot unsharing logic and modified node flushing logic.

Simplify descend* functions so that they no longer modify SHAMapNodeIDs. (They were too tricky to use because it wasn't clear which functions modified SHAMapNodeIDs and whether you were supposed to pass the parent or child ID.0


I'm seeing several places where we call some form of `fetchNode` and then `canonicalizeChild`.  It would be great if there were a method that did both so we wouldn't need to remember to call `canonicalizeChild` after `fetchNode`.  Just a thought.