This commit tigthen the public API for the `.CentralExchange` module,
so that it's clear which are the exported (public) functions, and
we do not leak the transport layer used by the central exchange

- edit bin/update-project-dependencies
- fix the values of :
  - sha256sum cabal.project
  - sha256sum cabal.project.freeze
- nix-shell $ ./bin/update-project-dependencies
- git add -u
- git commit

Also, fix for notifications throttle delay value

Also, throttle messages to avoid flooding the client