This avoids having to manipulate config files in brittle bash
entrypoints that need to react to dynamic service discovery.
This significantly improves the operability of various bench sites.
* refactor!: Drop currentsite.txt
- `bench use` will continue to work.
- Instead of txt file use common_site_config to set default site using `default_site` key.
- `FRAPPE_SITE` environment variable also works
* fix(DX): warn if non-empty currentsite.txt is present
- realtime communication uses pub-sub and no storage. So using same
redis server for both should be just fine.
- This is how FC works since quite a lot of time. We haven't seen any
problem so far.
