Added private method to TskImgDBPostgreSQL that looks for an existing module based on its name and returns the module id. This method is called by TskImgDBPostgreSQL::addModule() to initially check if the module already exists. If the module does not exist we attempt to insert. This insert attempt may fail if another node in a distributed environment creates the module entry. In that case we again fall back on the new getModuleId() method.