Added private method to TskImgDBPostgreSQL that looks for an existing module...

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.