Improve path+name handling in tree package

The DefinitionNodeImpl and ConfigurationNodeImpl classes handle path and name properties differently, leading to two related problems:

1. DefinitionNodeImpl constructors are unclear and redundant, since the final segment of the path already contains the name, which is also passed as a separate parameter. This appears to be just a simple design mistake.
2. ConfigurationNodeImpl#getPath() has a performance problem because it rebuilds s JcrPath objects on each call, where s is the number of segments in the path. This means an O[n] search of paths generates O[n*s] objects of garbage, when it should create none.