Some llama-run cleanups

[ericcurtin]

Use consolidated open function call from File class. Change read_all to to_string(). Remove exclusive locking, the intent for that lock is to avoid multiple processes writing to the same file, it's not an issue for readers, although we may want to consider adding a shared lock. Remove passing nullptr as reference, references are never supposed to be null. clang-format the code for consistent styling.