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


This would be another great place for an enum replacement for the bool.  If you read the calls to walkSubTree it is not at all obvious that the true or false mean doWrite or dontWrite.