Get rid of (most of) calls to `Lwt.async`

General context

From the Lwt documentation:

Lwt.async is misleadingly named. Itself, it has nothing to do with asynchronous execution. It's actually a safety function for making Lwt programs more debuggable.

Specifically, Lwt.async wraps a promise in a form of ignore and an exception catching mechanism that delegates the handling to a hook registered by a side-effect-ful function.

Tezos-specific context

We use the handler system from Lwt.async in the Lwt_exit library (src/lib_stdlib_unix/lwt_exit.ml). This sets the binaries to exit when an exception is thrown. This is the correct thing to do for some exceptions (e.g., the exceptions thrown by signal handlers), but it is not always desirable.

There are multiple occurences of Lwt.async within the source code.

Problem

Some simple errors could cause a node to exit.

Solution

Provide a async-like mechanism and replace uses of Lwt.async by said mechanism.