Add support manual create logical replication slot

[Lychangin]

@sgotti please review the pull request. This feature will be useful for example:

• Upgrade postgres cluster without downtime
• Logical replication for a separate database

[sgotti]

@Lychangin Thanks for your PR. As I explained here I'll prefer to simplify the current logic to namespace these additional repl slots with stolon_ like already done by the repl slots used for replication by standbys. I created #531, please take a look.

While your patch is excluding logical replication slot adding more undocumented logic, in #531 I just made all the repl slots prefixed with stolon_ so users will be able to manually manage every repl slots (it just shouldn't start with stolon_).

[Lychangin]

Thanks for the answer. I looked at your PR, I'm completely satisfied with it, I'll close my PR then.