Thread local caching of GcsIO instances does not work

gcsio.py contains some code to cache new instances per thread:
 
```

local_state = threading.local()
if getattr(local_state, 'gcsio_instance', None) is None:

```

However, since local_state is always a newly created thread-local storage, this property will never be set.

To get the intended effect, a global threading.local() instance should be stored on the class or module level.

Imported from Jira [BEAM-5206](https://issues.apache.org/jira/browse/BEAM-5206). Original Jira may contain additional context.
Reported by: hakon.

Start in gcsio.py by reading the thread-local caching code and checking how GcsIO instances are created. Confirm the storage for the thread-local state is shared at module or class scope, then verify that instances are reused within one thread and remain separate across threads.

Written by the indexing model from the issue text.