mypy: Finalize migration of cache.py to python3 function annotation.

- Use forward declarations of some types from models.py to avoid cycles.
- Remove cache.py from linter rule exclude list to ensure it stays that way.

tools/linter_lib/custom_check.py

@@ -477,7 +477,6 @@ def build_custom_checkers(by_lang):
              # Zerver files that we should just clean.
              'zerver/tests',
              'zerver/lib/api_test_helpers.py',
-             'zerver/lib/cache.py',
              'zerver/lib/request.py',
              'zerver/views/streams.py',
              # thumbor is (currently) python2 only