Layer1: changed Lwt_main.run calls into Tezos_base_unix.Event_loop.main_run

What

Prepare the code for libbees integration.

Why

Facilitate the switch to the Hive module, and also move from lwt to something else.

How

1. Add a Event_loop module to lib_base_unix library with a let main_run = Lwt_main.run
2. Use Event_loop.main_run instead of Lwt_main.run when relevant

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR