Add Concurrency Functions

Changed:
-save/saveNode: $includeNestedSetAttrs parameter that defaults to
false.  When false, Update statements will have PrimaryKey, Left,
Right, Level, and Root (if $hasMultipleRoots is true) attributes removed

Added:
-_lockAndRefresh(): lock the trees that are being updated inside of the
transaction using "SELECT…FOR UPDATE".  refresh nodes that have
attributes used in moveAsRoot(), addNode(), moveNode(), and delete()
functions.
-_calculateKey(): calculate the key of the node to append or move once
the dbTransaction has been locked and node refreshed