Loading Ecaml breaks the echoing of unfinished keystrokes

By default, Emacs displays unfinished keystrokes in the echo area if the user types slow enough. This behavior is broken when loading Ecaml. The problem seems to come from the initialization code of Ecaml, more precisely the start of the `Async` scheduler:

https://github.com/janestreet/ecaml/blob/68fa60749bcf25ea2c1378a8fe2da0eb4f53bf45/src/async_ecaml.ml#L692

Moreover, shutting down the scheduler via `M-x ecaml-async-shutdown` restores the echoing of unfinished keystrokes.

The problem was observed using the latest version of both Ecaml (v0.15.0) and Emacs (28.2).

Start at src/async_ecaml.ml around line 692, where the Async scheduler is initialized, and reproduce the issue in Emacs 28.2 with Ecaml v0.15.0 by typing unfinished keystrokes slowly. Compare the behavior while the scheduler is running with the behavior after M-x ecaml-async-shutdown. Done means Emacs again echoes unfinished keystrokes without requiring scheduler shutdown.

Written by the indexing model from the issue text.