Increase incremental parsing performance of cppmetrics

[mcserep]

Currently the CppAstNodeMetrics table is cleaned up during incremental parsing by iterating through all stored entities, querying the related CppAstNode, then the appropriate file. After that, CppAstNodeMetrics entities related to a deleted or modified file are erased.

I propose a likely more memory and time efficient approach: since the CppFileMetrics table is also maintained through the same logic (looking for entities related to a modified or deleted file), we could erase the related CppAstNodeMetrics entities in the same loop. (Probably a new ODB view is required to connect the CppAstNodeMetrics table to the files.)