Resource leak in CloudWalUploader due to file handles not being closed

[ethiebautgeorge-nasuni]

There are 2 potential resource leaks in CloudWalUploader due to file handles not being closed:

• file_object is not closed
• wal_file is not closed

When CloudWalUploader is run in a one-shot call from the CLI, the leak is not noticeable.
However, when run in sequence from a single process, if the Python gc doesn't trigger in a while, then the handles are leaked.
In our case, this leads to a handle exhaustion in our WAL uploader service.

PR: #1129

[ethiebautgeorge-nasuni]

@martinmarques @barthisrael Please have a look.

[barthisrael]

@martinmarques @barthisrael Please have a look.

Thanks for the PR!