At level 64, only leaves are allowed
Dispatch incoming TX set data
SHAMapNode and Ledger modification improvements
* Track dirty nodes in a set
* Make flushDirty a member function
* Don't write deleted nodes
* Make sure new nodes are shareable
* Put new nodes in the treeNodeCache
* Fix bug in SHAMap::canonicalize
* Push newly-created nodes into the tree node cache
* Try to retain nodes made resident by client requests