Incremental support for changes in compilation commands

[mcserep]

CodeCompass is already capable of detecting modified and deleted files, cleaning them from the workspace database, thus providing an incremental parsing functionality.

This PR extends incremental parsing to detect changes in the compilation commands. This is achieved as follows:

1. The compile commands from the given JSON file(s) are loaded.
2. All BuildAction entities are loaded from the workspace database.
3. For each BuildAction in the workspace database it is checked: if a BuildAction stores a command, which is not present in the compile commands database loaded from the JSON file(s), the source files for that build action are marked for cleanup. (This will ultimately clean up the given source file, the related AST info and the related build action itself.)