[MAT-388] Merge stages of expression translation

Now that cleanup is handled by shared_ptr, there is no need to
separate the two stages of expression translation in to traversal
of the Java tree and construction of the RDAG tree.
[MAT-388] Delete local refs of OGExpr nodes

After tree construction OGExpr local refs are no longer required,
so we can delete them. We cannot delete local refs for terminals,
because they're required at the end of execution for unbinding
arrays.
Merge pull request #110 from OpenGamma/MAT-388

Delete local refs of expression nodes as soon as possible.