SHAMap performance improvements (RIPD-434)
 * Remove mTNByID
 * Remove mDirtyNodes
 * Much faster traverses
 * Faster snapshots
 * New algorithm for flushing
 * New vistNodes/visitLeaves
 * Avoid I/O if a map is unbacked
[FOLD] SHAMap performance change fixes
* Missing canonicalize calls
* Can't canonicalize if seq!=0
* Adjust asserts
* Descend cleanups/simplifications
* Fix snapshot logic
[FOLD] fixes based on review
[FOLD] Use stacks based on vectors