Description

Fixes: #9066

Currently an underscore is appended to result_backend_transport_options['global_keyprefix']. This has at least two downsides:

1. the user cannot choose a separator, e.g. they may want to use : instead of _ (which is common in redis, for example)
2. this doesn't match documentation, which indirectly implies that a user needs to include their preferred separator:

app.conf.result_backend_transport_options = {
    'global_keyprefix': 'my_prefix_'
}

Changes:

• if the prefix ends with a separator (one of :_-.), use the prefix as is, otherwise append a _ separator for backward compatibility,
• add tests for new behavior.